Choc Marble Cake

INGREDIENTS:
½ cup unsalted butter, plus more for pan
1¾ cups cake flour (not self-rising)
2 tsps baking powder
½ tsp salt
1 cup sugar
3 large eggs, room temperature
1 tsp pure cocoa powder
½ cup buttermilk, room temperature
¼ cup plus 1 tablespoon Dutch-process cocoa powder

DIRECTIONS:
1. Preheat oven to 350 degrees. Generously butter a 9 by 5 inch loaf pan, set aside. Whisk together the cake flour, baking powder, and salt; set aside.

2. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy, about 5 minutes. Add eggs, one at a time, beating until combined after each addition and scraping down the sides of the bowl as needed. Mix in chocolate. Add flour mixture in 2 batches, alternating with the buttermilk and beginning and ending with the flour. Set aside ¼ of the batter.

3. In a bowl, mix cocoa and ¼ cup plus 2 tbsps boiling water with a rubber spatula until smooth. Add the cocoa mixture to the reserved cake batter; stir until well combined.

4. Spoon batters into the prepared pan in 2 layers, alternating spoonfuls of vanilla and chocolate to simulate a checkerboard. To create marbling, run a table knife through the batters in a swirling motion.

5. Bake, rotating the pan halfway through, until a cake tester comes out clean, 40 to 50 minutes. Transfer pan to a rack to cool 10 minutes. Turn out cake from pan and cool completely on the rack. Cake can be kept in an airtight container at room temperature up to 3 days.