Preheat the oven to 325 degrees F. Using 3, 8 inch cake pans, spray the bottom of the cake pan with cooking spray, line with parchment paper, and spray again. Set aside.
In a blender, pulverize the Biscoff cookies to resemble a fine crumb.
In a medium sized bowl, whisk the melted butter, sugar, and pulverized Biscoff cookies until they are evenly combined. You should be able to grab the mixture by hand it have it stick together.
32 Biscoff cookies, ⅓ cup granulated sugar, ½ cup unsalted butter
Pour about ¾ cup of the mixture into each prepped cake pan. With a flat surface, firmly and evenly press the Biscoff mixture into each pan, making sure to get it in the corners too. Bake for 8 minutes. Let them cool for 10 minutes before putting the batter into them.
While the layers are baking, start making the Biscoff cake batter.
For the Biscoff cake
Place the Biscoff cookies in a blender and blend until the cookies are finely ground into a fine crumb.
In a medium-sized bowl, whisk together the Biscoff crumbs, all-purpose flour, baking powder, baking soda, and salt until evenly combined.
In the bowl of a stand mixer fitted the paddle attachment, cream the butter, brown sugar, and oil on medium speed until light and fluffy, about 5 minutes. The mixture will start looking light in color.
1 cup unsalted butter, 1 ⅔ cups light brown sugar, ¼ cup vegetable oil
Add the eggs, one at a time, beating on medium speed for 30 second after each addition. Add the vanilla and mix on the lowest speed, just until combined.
4 large or extra large eggs at room temperature, 1 teaspoon vanilla
With the mixer on the lowest speed, add ⅓ of the dry ingredients and half of the buttermilk. Mix just until combined. Repeat this step. Add the rest of the flour mixture and continue to mix on the lowest speed until the flour and the buttermilk have all been incorporated. With a rubber spatula scrape down the sides and bottom of the bowl and slowly mix by hand.
2 cups buttermilk at room temperature
Divide the batter evenly among your pans, (about 20 ounces in each pan) and bake for about 35-39 minutes. To check, insert a toothpick. Once little to no crumbs come out on your toothpick, the cakes are done. (my oven runs a little cooler than most, so mine took 39 minutes) Because of the cookies and the brown sugar in this recipe, the cake layers will brown on top. That is normal and what you want.
After they are out of the oven, I let mine cool for 15 minutes. Then I transfer them onto a wire rack to finish cooling. Let them come to room temperature before wrapping them in plastic wrap and freezing or refrigerating.
For the cookie butter filling
In a stand mixer fitted with a paddle attachment, beat the butter and Biscoff spread, cream, powdered sugar, and salt on medium speed until light and fluffy, about 3 minutes.
½ cup unsalted butter at room temperature, 1 ½ cups Biscoff cookie butter spread, 1 ½ cups powdered sugar, 2 Tablespoons heavy cream, tiniest pinch of salt
For the vanilla buttercream
In a bowl fitted with a paddle attachment, beat the butter on medium-high speed for about 3 minutes.
2 cups unsalted butter, 6 cups powdered sugar, ⅓ cup heavy whipping cream, 2 teaspoons pure vanilla extract, ½ teaspoon salt
Add powdered sugar, vanilla extract, salt and heavy cream. Mix on low. Once incorporated, beat on high for 4-5 minutes.
Before frosting your cake, use a wooden spoon or rubber spatula to hand mix the frosting to push out any air bubbles.
Assembly
Smear a dollop of frosting in the middle of the cake board to act as a glue. Place the first cake layer on a cake board, with the Biscoff crust layer on the bottom. Gently and evenly spread half of the cookie butter filling, followed by a some melted cookie butter. (The melted cookie butter is optional)
Repeat with the second layer of cake.
Place your final cake layer, with the Biscoff crust on the bottom, right on top of the second layer of filling.
Apply a thin layer of the vanilla buttercream around the entire cake and freeze the cake for 10 to 15 minutes.
Once the crumb coat is set, finish frosting and decorating the cake with the remaining vanilla buttercream. Garnish with Biscoff cookie crumbs.