Prepare the Cake Layers:

1. Preheat the oven to 350°F (175°C). Line three 8-inch cake pans with parchment paper and grease them well.
2. In a large bowl, mix flour, granulated sugar, brown sugar, baking powder, cinnamon, and salt.
3. Add the softened butter and mix until the mixture resembles coarse crumbs.
4. In a separate bowl, whisk together egg whites, buttermilk, vegetable oil, and vanilla extract.
5. Gradually add the wet ingredients to the dry ingredients, mixing until smooth.
6. Divide the batter evenly among the prepared pans and bake for 33-35 minutes or until a toothpick inserted into the center comes out clean.
7. Cool the cakes in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

2. Make the Vanilla Buttercream:

1. Beat the butter on high speed until creamy and fluffy (about 2 minutes).
2. Gradually add powdered sugar, vanilla extract, and salt, mixing on low speed.
3. Slowly pour in the heavy cream, then increase to high speed and beat until light and fluffy.

3. Create the Cookie Butter Buttercream:

1. Add 1 cup of cookie butter to half of the prepared buttercream.
2. Stir in ¼ cup heavy cream for a smoother texture, if needed.

4. Assemble the Cake:

1. Level the cake layers using a serrated knife to ensure even stacking.
2. Place the first cake layer on a cake board or serving plate. Spread a layer of cookie butter buttercream, followed by a thin layer of cookie butter.
3. Repeat with the remaining cake layers.
4. Apply a thin crumb coat of buttercream around the cake and refrigerate for 30 minutes.
5. Frost the cake with the remaining buttercream.

5. Decorate:

1. Drizzle cookie butter over the top of the cake.
2. Decorate with crushed or whole Biscoff cookies around the sides and top.
3. Optionally, pipe buttercream swirls on top for an elegant finish.