Melt 2 cups of butter in a medium saucepan over medium heat. Stir it frequently as it cooks. After a few minutes, the butter will turn a golden brown color and give off a nutty aroma. Remove it from the heat and let it cool slightly. You’ll need about ¾ cup of this brown butter for the cake batter.

Preheat your oven to 350°F (175°C). Prepare your cake pans by either greasing and flouring them or lining them with parchment paper.

In a large mixing bowl, combine the flour, sugar, baking powder, and salt. Whisk them together thoroughly and set aside.

In a separate bowl, mix together the remaining brown butter, vegetable oil, sour cream, vanilla extract, and egg whites. Whisk until the mixture is smooth and fully combined.

Gradually add the dry ingredients to the wet mixture, folding gently until everything is incorporated. Take care not to over mix the batter.

Divide the batter evenly between your prepared pans. Bake for 40–45 minutes, or until a toothpick inserted into the center of the cake comes out clean. Let the cakes cool in the pans for about 15 minutes before transferring them to a cooling rack to cool completely.

In a large bowl, beat together softened butter and cookie butter until creamy. Slowly add the powdered sugar, cinnamon, and salt, mixing until smooth. Finally, add the heavy cream and vanilla extract and continue mixing until the buttercream is light and fluffy.

Once the cakes are fully cooled, spread a layer of the buttercream between the layers, then cover the top and sides of the cake. Decorate as you like and serve.