Ingredients
1/2 cup (1 stick) unsalted butter, at room temperature
1 + 3/4 cups cake flour
2 teaspoons baking powder
1/2 teaspoon salt
3/4 cup granulated sugar
3 large eggs, at room temperature
1 teaspoon pure vanilla extract
2/3 cup buttermilk, room temperature
1/4 cup plus 1 tablespoon cocoa powder
Preparation
Preheat oven to 350°F. Spray a 9-by-5-inch loaf pan with cooking spray or line with parchment paper; set aside.
Whisk together the cake flour, baking powder, and salt; set aside.
Beat together the butter and sugar until light and fluffy, about five minutes. Add eggs, one at a time, beating until combined. Mix in vanilla. Add flour mixture in two batches, alternating with the buttermilk and beginning and ending with the flour.
Set aside 1/3 of the batter.
In a small bowl, mix the cocoa with 1/4 cup plus 2 tablespoons boiling water with a rubber spatula until smooth. Add to the batter set aside and stir until well combined.
Spoon batters into the prepared pan, alternating between vanilla and chocolate. To create a marbling effect, cut a knife through the batters in a swirling motion.
Bake for 40-50 minutes, rotating the pan halfway through, until a toothpick inserted into the center of the cake comes out clean. Allow to cool 10 minutes, then turn cake out from pan and allow to cool completely.
