Preheat the oven to 350°F (175°C) and prepare two 9-inch round cake pans with parchment paper and non-stick spray.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, brown sugar, baking powder, ground cinnamon, and fine salt.

Add the unsalted butter to the dry ingredients and mix until the texture resembles coarse crumbs. Then, add the egg whites and buttermilk, stirring until smooth.

Divide the batter evenly between the prepared cake pans and bake for 33–35 minutes or until a toothpick comes out clean.

Let the cake layers cool in the pans for about 10 minutes, then transfer to a wire rack to cool completely.

Beat the unsalted butter until smooth and creamy, then gradually add the powdered sugar and a splash of heavy whipping cream until desired consistency is achieved.

Place one cake layer on a serving plate, spread frosting over it, then add the second layer and frost the top and sides.

Drizzle melted cookie butter on top and decorate with chopped Biscoff cookies, if desired.