Marble Cake

INGREDIENTS

2 cups

cake flour (spoon & leveled)1

2 tsp

baking powder

1/2 tsp

salt

1 cup

(230g; 2 sticks) unsalted butter, softened to room temperature

3/4 cup

granulated sugar

1/2 cup

tightly packed light brown sugar

large eggs, at room temperature

large egg yolks, at room temperature

1 tbsp

pure vanilla extract

2/3 cup

buttermilk, at room temperature2

4 oz

bittersweet or semi-sweet chocolate, coarsely chopped3

and 1/4 cups (284g; 2.5 sticks) unsalted butter, softened to room temperature

and 1/2 cups (420g) confectioners' sugar

3/4 cup

natural unsweetened or dutch-process cocoa powder

1/4 tsp

salt

1 tsp

pure vanilla extract

1/4 cup

heavy cream, half-and-half, or whole milk

optional: chocolate or rainbow sprinkles for decorating