The brown butter adds a subtle nutty flavor to a simple vanilla cake. Top with dark chocolate buttercream for a classic combo!
In a small, light-colored saucepan melt butter over medium heat, stirring frequently until the butter starts to brown and smell nutty. Transfer to a heat-proof bowl (use a rubber spatula to get those brown flecks!) and chill in the freezer for about 20 minutes. Once solid, remove and let it warm up to room temperature.
Preheat oven to 350F. Spray 5 6" cake pans with cooking spray (you could also use two 8" round pans). Set aside.
In the bowl of a standing mixer fitted with the paddle attachment, combine flour, sugar, baking powder and salt on low speed until combined. Gradually add the butter, one spoonful at a time. Mix on low until combined. Add egg whites and continue mixing on low speed. Pour half of the buttermilk in, mixing until combined. Add remaining buttermilk, vanilla and oil. Turn up to medium speed and mix for an additional 30 seconds.
Evenly distribute between prepared cake pans. If using smaller pans, bake for 15 - 17 minutes (about 35 minutes for larger pans) until a toothpick inserted into the middle comes out clean. Remove from oven and let cool for about 10 minutes, then remove from pans and transfer to a wire rack to finish cooling.
In the bowl of a standing mixer fitted with the paddle attachment, beat butter on medium speed until smooth and creamy. Add cocoa powder and 1 cup of powdered sugar. Mix on low and alternate adding the remaining powdered sugar and heavy cream until you reach the desired consistency. Add vanilla and pinch of salt, then mix on medium-high for an additional minute.
Spread an even layer on top of one cake layer, then repeat until cake is full stacked. Frost the top and sides, using a bench scraper to get it extra smooth. Enjoy!