In a medium bowl, whisk together the flour, baking powder and salt until they're evenly distributed.

Place the room temperature butter and the granulated sugar in a large bowl and use an electric mixer with the whisk attachment to beat them for 2 minutes, scraping the edge of the bowl halfway through.

Add in the oil and beat for another 1-2 minutes, until it's pale and fluffy.

Add the egg whites, 1-2 at a time, beating on medium speed between each addition, until each egg is fully incorporated and the batter is fluffy.

Add the vanilla extract and sour cream and beat just until they're combined.

Add half of the dry ingredients to the batter, turn the mixer up to low speed and then pour in the cream, followed by the rest of the dry ingredients.

Fold in the chopped biscoff cookies just until they're evenly distributed.

Spread the cake batter in the prepared cake pans, about 15 ounces in each, and bake for 40 minutes.

When the cake is fully baked it will release itself from the edge of the pan and it will spring back if you press on the center of it.

Allow the cake to cool completely before frosting.

Place the sugar and water in a medium saucepan and swirl the pan until the sugar is fully immersed in water. Place the saucepan over medium heat until the sugar dissolves and comes to a gentle simmer. Lower the heat to low and place the lid on the pan and allow the mixture to simmer for a couple minutes.

Meanwhile, place the egg yolks in the bowl of a stand mixer fitted with the whisk attachment, and turn the mixer up to high speed. Let it whip while the sugar is done cooking.

Use a candy thermometer to see when the sugar reaches 240F (116C), then lower the mixer to medium speed and slowly drizzle the hot sugar syrup into the thick and foamy egg yolks, while they are whipping.

Turn the mixer back up to high speed and let it whip for 5-10 minutes, until the yolk mixture is very pale and fluffy and is cool to the touch or no longer warm.

Turn the mixer back down to medium speed and add in the butter, 2 Tbsp of butter at a time, mixing until each chunk is fully combined and the buttercream looks smooth and creamy.

Scrape the sides and bottom of the bowl, add in the vanilla, salt and cookie butter and turn the mixer up to high speed for 1-2 minutes.

Place the buttercream in a piping bag fit with a round piping tip.

Spread an even layer of buttercream on the first layer of cake and then pipe a border around the edge.

Fill the border in with a layer cookie crumbs and a generous drizzle of cookie butter spread. Refrigerate the cake for about 15-20 minutes so the buttercream can firm up before stacking the next layer.

Repeat with the second layer of cake. Refrigerate and then place the third layer on top.

Refrigerate the cake for 30 minutes minutes, or until it feels sturdy enough to frost the outside.

Frost the whole cake with the cookie butter buttercream, reserving just a little to pipe poofs and then refrigerate the cake just until the buttercream feels set, about 30-60 minutes.

Place a sheet of parchment over the top of the cake, leaving a little bit exposed, and use a sieve to dust some leftover biscoff cookie crumbs on top of the exposed area of the cake.

Use the leftover buttercream to pipe little poofs randomly on the cake with a fine tooth star piping tip. 

Then lightly melt some of the biscoff cookie butter spread and place it into a small piping bag with a small round piping tip. Randomly pipe small dots on the cake.

Refrigerate for at least 30 minutes to let the buttercream set fully. Serve slightly chilled or at room temperature.