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.
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.
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.
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.
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 apple filling
In a large bowl, combined all ingredients except the corn starch and water. Toss until evenly combined.
Place the apples mixture in a medium-large pot over medium-high heat. Slowly bring to it to boil over, using a rubber spatula to stir. Make your slurry by add cold water to the corn starch. Set aside
As the apples begin cook they release liquid. Once apple have slightly cook and look slightly soften, Add the slurry to the mixture stirring constantly. It should start to thicken after about 30 seconds. If it hasn’t, make a little more slurry.
Once done, remove the apple and transfer to a container and put in the fridge to cool down. Once completely cooled, the filling can be stored in the fridge in an airtight container for up to 1 week.
For the Buttercream
In the bowl of electric mixer fitted with the paddle attachment, combine the butter and cookie butter spread. Beat on medium-high speed until smooth, about 45 seconds.
Add the powdered sugar to the bowl and mix in, slowly at first until incorporated, then increasing the mixer speed to high. Blend in the vanilla extract, salt and heavy whipping cream, and continue to whip on high speed until very fluffy, about 4-5 minutes, scraping down the sides of the bowl as needed.
Beat frosting by hand with a wooden spoon to get air bubbles out.
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.
Pipe a rim of frosting around the edge of the cake to create a barrier for the filling.
Add ½ cup of the apple filling to the top of the cake layer, evenly spread it across the cake.
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 cookie butter 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 buttercream. Garnish with Biscoff cookie crumbs.