Birthday Sheet Cake (Party Size)

Preheat the oven to 350F/177C and grease and two 13x18" sheet pans (can do one at a time).
In a medium bowl, whisk together the flour, baking powder and salt until they're evenly distributed.
540 grams all-purpose flour spooned and leveled, 2 Tbsp baking powder, 1 tsp fine sea salt
Place the room temperature butter, the oil and the granulated sugar in a large bowl and use an electric mixer with the whisk attachment to beat them for 2 minutes.
113 grams salted butter room temperature, 240 mL canola oil, 600 grams granulated sugar
Add the eggs, one at a time, beating on medium speed until each egg is fully incorporated. Add the sour cream and vanilla extract and beat just until combined.
6 large eggs room temperature, 340 grams sour cream room temperature, 2 Tbsp vanilla extract
Add half of the dry ingredients to the wet ingredients, turn the mixer on low speed and then pour in the milk, followed by the rest of the dry ingredients.
360 mL milk room temperature
Mix in the sprinkles and make sure to scrape the bottom and edge of the bowl well before spreading into pan.
150 grams jimmy or confetti rainbow sprinkles
Distribute the batter evenly among the two cake pans, about 44oz in each pan and bake for 20-25 minutes. When the cake is fully baked it will spring back if you press on the center of it.
Allow the cake to cool completely before frosting.

In a medium bowl, combine the egg yolks, sugar, salt and cornstarch. Whisk them together until the mixture is smooth; set aside.
6 large egg yolks room temperature, 100 grams granulated sugar, 1/4 tsp fine sea salt, 60 grams cornstarch
In a medium saucepan, over low-medium heat, bring the milk to a simmer and then slowly stream the milk over the egg mixture, while whisking continuously.
480 mL milk
Return the whole mixture to the pot and over low-medium heat, whisk continuously for a 1-3 minutes until it starts to thicken.
Now whisk aggressively for another 30-60 seconds and then remove the cream from the heat and pour into a bowl (pour through a sieve if it's lumpy)
Add the butter and vanilla and allow it to melt from the heat of the pastry cream. It'll separate at first but as you whisk it, it'll come together and be nice and smooth.
84 grams salted butter softened, 1 Tbsp vanilla bean paste
Cover the surface of the custard with plastic wrap and allow to cool to room temperature.

Place the egg whites and granulated sugar in the bowl of a stand mixer. Bring a small pot with 1/2 cup of water to a boil and lower the heat to simmer.
8 large egg whites room temperature, 400 grams granulated sugar
Place the bowl of egg whites and sugar on top of the simmering water for 5-10 minutes, whisking intermittently, until the granulated sugar is completely dissolved.
Once the sugar is dissolved, remove the bowl of egg whites from the heat and use an electric mixer with a whisk attachment to beat the egg whites with the sugar. Beat for 7-10 minutes on high speed and then lower the speed to low-medium.
Slowly add 2 Tbsp of butter in at a time. Wait for the butter to incorporate before adding the next chunk. Once all the butter is added, turn the mixer up to high speed for 10-15 seconds.
454 grams unsalted butter room temperature
Use a rubber spatula to scrape the edge of the bowl, add the vanilla bean paste (or extract) and salt and beat until they're fully combined.
2 Tbsp vanilla bean paste or extract, 1/4 tsp salt

Once everything has cooled and you're ready to assemble, beat the heavy cream for the custard filling, until it reaches stiff peaks.
Gently whisk the pastry cream to break it up a bit and then fold the heavy cream into the pastry cream to make it a little lighter.
Place the first layer of cake onto a flat surface. I recommend a large cake board.
Use a piping bag fit with a round tip to pipe a border of the buttercream around the edge of the cake layer. Then fill in the center with the custard cream filling
I slice the top layer of cake in half so it's easier to transfer. Place it on top of the frosting and cream filling and refrigerate the cake for 20-30 minutes to set before frosting the outside.
Use the rest of the buttercream with an offset spatula and cake scraper to frost the outside of the cake. Top with sprinkles, candles, and whatever else you like!