. 1/2 cup unsalted butter, room temperature, plus more for the pan
. 1 cup sugar
. 1 3/4 cups flour
. 1 1/2 teaspoons baking powder
. 1/2 teaspoon salt
. 3 large eggs
. 1 1/2 teaspoons vanilla
. 2/3 cup buttermilk
. 1/3 cup cocoa powder
. Preheat the oven to 350°F. Grease and flour the loaf pan.
. Cream the butter and sugar. Using a stand mixer with a paddle attachment, beat the butter and sugar until light lemon-colored and fluffy, about 5 minutes.
. Measure the flour and crack the eggs. While the butter and sugar are creaming, measure out the flour, baking powder, and salt into a small bowl and whisk lightly to combine. Crack the eggs into a bowl and add the vanilla.
. Add the eggs. Scrape down the sides of the bowl with a spatula. Add the eggs, one at a time, beating well in between each addition and scraping the bowl as needed.
. Add the flour and buttermilk, alternating. Add 1/3 of the flour to the batter and beat on low speed just until incorporated. Then add 1/2 of the buttermilk and beat.
. Add another 1/3 of the flour, and beat, and then the last of the buttermilk and beat. Finally, beat in the last 1/3 of the flour.
. Add the chocolate. Transfer roughly 1/2 of the batter to a separate bowl.
. In a small bowl, mix the cocoa powder with 2 tablespoons of hot tap water until smooth. Add this chocolate mixture to the remaining batter.Beat to incorporate.
. Spoon into the prepared pan, creating a two-layer .
. Beginning with the vanilla mixture, place four spoonfuls of batter into the pan, spacing them far enough apart so that you can create a checkerboard when you add the chocolate batter.
. Add the chocolate batter into the empty spaces to fill the bottom of the pan in a checkerboard pattern. Repeat with the second layer, alternating the vanilla and chocolate batter.
. Using the handle of a wooden spoon, swirl the batter to create a marbled effect.
. Bake the cake. Place in the oven and bake for about 40 minutes, rotating the pan half way through. The cake is done when it has browned slightly and a cake tester comes our clean when inserted into the middle.
. Cool on a rack for 10 minutes and then unmold and let cool completely.