Ingredients
Unsalted butter, for loaf pans
1/2 cup unsweetened cocoa powder
1 recipe Classic Yellow Cake Batter
Preparation
Preheat oven to 350 degrees. Butter four 2-by-6-inch miniature-loaf pans (2 inches deep; not nonstick). In a medium bowl, whisk together cocoa powder and 6 tablespoons hot water. Stir half the batter into cocoa mixture.
Spoon 2 dollops of batter into pan next to each other, 1 chocolate and 1 vanilla. Repeat twice, alternating layers, and swirl.
Bake until a skewer inserted in center of a cake comes out with a few moist crumbs attached, 40 to 45 minutes, rotating pans halfway through. Let cool completely in pans on a wire rack. Run a knife around edges, then turn out cakes.
