Preheat the Oven: Preheat your oven to 350°F (175°C) and grease three 8-inch round cake pans. Line the bottoms with parchment paper.
Mix the Dry Ingredients: In a medium bowl, sift together the all-purpose flour, baking powder, cinnamon, and salt. Set aside.
Whip the Egg Whites: In a large mixing bowl, whip the egg whites until soft peaks form. Gradually add the granulated sugar and continue beating until stiff peaks form.
Combine Wet Ingredients: In a separate bowl, mix the brown sugar, vegetable oil, buttermilk, and vanilla until well combined.
Incorporate Dry and Wet Mixtures: Gently fold the dry ingredients into the wet mixture. Once combined, fold in the whipped egg whites in three parts to keep the batter light and airy.
Bake the Cakes: Divide the batter evenly among the prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool completely before frosting.
Cream the Butter: In a stand mixer or large bowl, beat the softened butter until creamy and smooth.
Add the Cookie Butter: Mix in the cookie butter until fully combined.
Incorporate Powdered Sugar: Gradually add the powdered sugar, one cup at a time, beating well after each addition.
Adjust Consistency: Add the heavy cream, vanilla, and salt. Beat until the frosting is fluffy and spreadable. Adjust the consistency with more powdered sugar or cream, as needed.
Prepare the Layers: Level the tops of the cooled cakes with a serrated knife if necessary. Place one cake layer on a cake board or serving plate.
Layer the Frosting: Spread an even layer of frosting over the first cake layer. Repeat with the second and third layers.
Crumb Coat and Chill: Apply a thin crumb coat of frosting over the entire cake and refrigerate for 20 minutes to set.
Final Frosting and Decoration: Frost the cake with a smooth or textured final coat of frosting. Use a French piping tip to create decorative swirls or patterns on top. Garnish with crumbled Biscoff cookies and drizzle cookie butter (microwaved for easier pouring).
Whipped Cream Garnish (Optional): Whip 1/3 cup heavy cream with a pinch of cinnamon and pipe it onto the cake for an extra decorative touch.
Serve and Enjoy: Slice and serve this decadent cake to your guests, and watch it disappear in no time!