1/2 cup buttermilk, room temperature
4 large eggs, room temperature
2 teaspoons vanilla extract
2 1/4 cups all-purpose flour
1 1/2 cups granulated sugar
1 1/2 teaspoons baking powder
1/2 teaspoon baking soda
3/4 teaspoon salt
16 tablespoons (2 sticks) unsalted butter, cut into 16 pieces and softened
12 tablespoons (1 1/2 sticks) unsalted butter, cut into 12 pieces and softened
2 cups packed dark brown sugar
1/2 teaspoon salt
1/2 cup heavy cream
1 teaspoon vanilla extract
2 1/2 cups Confectioners' sugar, sifted