Vanilla Birthday Sheet Cake (Party Size)

This party-size cake is inspired by the classic Costco birthday cake. It’s made with two large vanilla cake layers (with or without sprinkles), a vanilla custard cream filling, and vanilla Swiss meringue buttercream.

The cake layers are baked in two large sheet pans so the cake is easy to fill, frost, slice, and serve for a crowd. 

The custard cream gives it that nostalgic bakery-style filling, and makes it a little more interesting than just vanilla frosting.

It’s basically the kind of cake you want for birthdays, graduation parties, holidays (hello red & blue sprinkles!), or anywhere you need a big vanilla cake that still feels homemade and special.

And of course, the best part is, you can customize it any way you like! 

The Details 

1. Vanilla Cake – I used my favorite base recipe from my 6-Inch Cake. This cake size bakes perfectly in a half sheet pan (13×18 or similar) as a thin layer. I doubled that recipe to make two layers and added sprinkles. 
2. Vanilla Custard Cream – I thought pastry cream would be a little bit too thick/rich for this so I lightened it up a little with whipped cream, similar to a Diplomat Cream but I used a little less cream and no gelatin. 
3. Vanilla Buttercream – I used my Swiss Meringue Buttercream recipe to frost the cake (but if you prefer it sweeter and less buttery, American also works!). 

Vanilla Cake

1. Beat together the butter and sugar for 1-2 minutes with the whisk attachment.
2. Add the oil and continue beating for another 1-2 minutes.
3. Add the eggs, one at a time, beating for at least 30-60 seconds between each addition, until each one is combined. Add the sour cream and vanilla and mix just until they’re incorporated.
4. Add in half of the dry ingredients, followed by the milk and then the other half of the dry ingredients. Mix just until the last streak of flour is combined.
5. Fold in the sprinkles, mixing just until they’re evenly distributed.
6. Distribute the batter evenly between two half sheet pans, about 45 ounces of batter in each one.
7. Bake for 15-20 minutes, until the cakes are golden brown, the edges released from the pan and/or the cake springs back when you press on it gently. Allow it to cool completely before removing from the pan.

Sprinkle Options

Not all sprinkles are made equally. For the topping, anything is fine but for mixing into cake batter, I recommend rainbow jimmies or rainbow sequins. Both of these are soft enough to melt into the batter but sturdy enough that they don’t bleed too much before baking.

I don’t recommend using nonpareils, they can bleed into the batter, creating either a tie dye effect or even a muddy color if they’re over-mixed. 

Vanilla Buttercream

Side Note: I find it annoying that after using 6 egg yolks for the filling, the buttercream needs 8 egg whites lol so if you’re anything like me and you don’t want left over egg yolks, here’s what you can do.

You can get away with making a Swiss meringue buttercream with only 6 egg whites, just do 75% of the full recipe (300 grams sugar, etc.) but it will just be enough to lightly frost the cake (similar to mine), you will not have enough to decorate it at all. 

If you do make the full amount and need to get rid of egg yolks, I have a chocolate chip cookie recipe (linked in the blog post) that I intentionally created to use up leftover yolks! 

1. Whisk egg whites and granulated sugar in the bowl of a stand mixer (I used a glass bowl for these photos but I don’t recommend it cuz it can break, use the aluminum mixer bowls), over a double boiler, until the sugar dissolves and/or reaches 170F (77C). 
2. Beat egg whites and sugar together at full speed until fluffy and no longer hot.
3. Add the butter to the meringue and mix – it will look soft and runny at first.
4. Increase speed and continue beating until the buttercream emulsifies. 
5. Add in vanilla and salt and mix just until they’re combined. 

Vanilla Custard Cream

I usually make the pastry cream right after the cakes, giving it ample time to cool. Then mix in the whipped cream right before assembling.

1. Heat the milk just until it starts to steam and/or simmer, 1-2 minutes.
2. Whisk together the eggs, sugar, cornstarch and salt until it’s completely smooth and there are no lumps.
3. Temper the eggs, by streaming the hot milk into the egg mixture while whisking the eggs. Pour the whole mixture back into the saucepan and continue whisking.
4. The pastry cream will begin to thicken after about 2-3 minutes; continue whisking aggressively for another minute or so.
5. Continue whisking while you remove the pan from heat and then dump it into a bowl. Give it another good whisk, making sure there are no lumps. It should be thick and spreadable, not pourable. 
6. Add the butter & vanilla and allow the heat from the pastry cream to melt the butter. It will separate at first but as you whisk, the pastry cream will come back together and should be smooth and silky. *if you have any lumps, feel free to press the pastry cream through a sieve while it’s still warm. 
7. Cover the surface of the pastry cream with plastic wrap and allow it to cool to room temperature. You can refrigerate it but if it firms up too much, gently whisk it before adding the whipped cream so that it’s not chunky.
8. Whip the cream just until it reaches stiff peaks.
9. Fold in the whipped cream, just until they’re fully combined.

Assemble

1. Place the first layer of cake onto a flat cake board. 
2. Pipe a border of the buttercream around the edge of the cake. 
3. Fill in the border with the vanilla custard cream.
4. Place the second cake on top and frost the outside of the cake with the buttercream. If the top layer of cake doesn’t feel sturdy when frosting, refrigerate the cake for 30 minutes before frosting the outside. 

How To Transfer Large Cakes When Assembling

I felt like I needed to include this because the first time I made it, I didn’t know how to pick up the cakes without breaking them. 

For the bottom layer, I usually flip it over onto a cutting board or any flat surface that’s big enough. Then flip it back over onto whatever board you’ll be serving it on. I recommend a cake board like this because the cake doesn’t fit well on a standard size cutting board. 

For the top layer, I sliced it in half and just transferred them separately to the top of the cake, since they get sliced eventually anyway. 

Pan Sizes & Servings

I baked this full cake in two half sheet pans which are 13”x18” (12”x17” from interior rim to rim). 

This cake serves approximately 48 small/medium slices. 

Any pan of similar size/square footage would work fine, just try not to make the cakes any thicker or thinner than I did. Thinner cakes would fall apart more easily and thicker cakes would squish the custard cream filling. 

You can also halve the recipe, bake it in one half sheet pan, slice it in half and use that to make a two layer cake that serves about 24 small slices. 

Slice sizes are the exact ones in my photo if that gives a better reference to size. 

How To Customize A Party Cake

Of course you can decorate the cake as desired for any event but I figured some flavor options would be fun. 

I always recommend piping the border, frosting and decorating with Swiss Meringue Buttercream or American Buttercream. You can also skip the custard filling and use the frosting to fill the center as well but I would double the recipe then. 

Feel free to change the flavor of the frosting or use any of my recipes that are made for 6-inch cakes and bake it this way for a party size cake.

For example, my Chocolate Cake would work exactly the same or you can make an Oreo party cake, Marble, Chai, Fruit Pebble, Nutella, you get the point – just double those recipes and use them the same I did here. 

Serving & Storing

Store the cake covered in the refrigerator because of the custard cream filling. 

I like to bring it to room temperature about 30-60 minutes before serving so the buttercream and cake aren’t too firm. But also the cake slices cleanest when cold, so don’t let it get too soft before slicing. 

The assembled cake is best within a couple of days. For longer storage, freeze slices or leftover cake as soon as possible to keep it fresh.

Thank you so much for trusting me with your celebration cake. If you have any questions, comment below, and please leave a 5- star rating or review if you make it!

As always, have a blessed day and happy baking! 

Love, B