Brown that butter: Melt butter over medium heat until golden and nutty-smelling. Remove from heat and let cool to room temp.
¾ cup 170g unsalted butter
Preheat oven to 350°F (175°C). Grease and line cake pans.
In a bowl, whisk flour, baking powder, salt.
2 ½ cups 300g all-purpose flour, 2 ½ tsp baking powder, ½ tsp salt
In another bowl, beat cooled brown butter + sugar until fluffy.
1 ¾ cups 350g granulated sugar
Add eggs one at a time, beating well. Mix in vanilla.
4 large eggs, 1 tbsp vanilla bean paste
Mix in half dry ingredients, then milk + sour cream, then remaining dry.
1 cup whole milk, ¼ cup sour cream or Greek yogurt
Divide into pans and bake 30–35 min, or until toothpick comes out clean.
Cool completely before layering.