Instructions

• Prepare the pan: Spray a loaf pan with baking spray or butter it, then sprinkle a little flour and tap out the excess. Cut a strip of parchment paper to fit the pan with overhanging edges on the long sides and place it in the pan (overhang will help remove the cake later).
• Preheat the oven to 300°F (150°C).
• Melt the chocolate: put the chopped chocolate in a dry, medium bowl set over a pan of barely simmering water (bain-marie). Stir gently until smooth. Remove from heat but keep the bowl over the warm water or off the heat to stay warm while you continue.
• Soften the dulce de leche: put the dulce de leche in a microwave-safe bowl and heat about 30 seconds, until soft and fluid. If you don’t have a microwave, warm it gently in a small saucepan until pourable, then transfer to a medium bowl.
• In the bowl of a stand mixer fitted with the paddle attachment, beat the room-temperature butter and sugar on medium-high speed until light and fluffy, about 5 minutes. Stop once or twice to scrape down the sides of the bowl.
• With the mixer on medium speed, add the eggs one at a time, beating briefly after each addition and scraping down the bowl each time so the mixture is even.
• In a small bowl, whisk together the flour, salt, and baking powder.
• Fold the dry ingredients and sour cream/buttermilk into the butter mixture in three stages to avoid overmixing: (a) add one-third of the dry ingredients and stir just until almost incorporated; (b) stir in half of the sour cream or buttermilk; (c) add half of the remaining dry ingredients and mix just until nearly incorporated; (d) stir in the remaining sour cream or buttermilk; (e) add the final dry ingredients and stir just until everything is combined. Do not overbeat.
• Divide the batter into three equal portions (by weight if possible, otherwise by eye).
• Make the chocolate batter: combine one portion of batter with the melted chocolate and the 1 tablespoon Dutch-process cocoa powder in a bowl; stir until evenly combined.
• Make the dulce de leche batter: combine a second portion of batter with the warmed dulce de leche in a bowl; stir until evenly combined.
• Make the vanilla batter: stir the 2 teaspoons vanilla extract into the remaining portion of batter left in the mixing bowl.
• Layer the batters in the prepared loaf pan: using a scoop or large spoon, place alternating scoops of chocolate and vanilla batter in five parallel rows running lengthwise along both sides of the pan (chocolate and vanilla alternating). Place scoops of the dulce de leche batter down the center of the pan between those two sets of rows. Tap the pan lightly on the counter to settle the batters.
• Add a second layer to create a checkerboard/marble effect: place scoops of chocolate batter over the vanilla scoops, place dulce de leche scoops over the chocolate scoops, and finish with vanilla scoops down the center as space allows. Place any remaining batter on top to fill the pan. Tap the pan on the counter to eliminate air pockets.
• Swirl the batters: run a toothpick or butter knife through the batter 6–7 times lengthwise to lightly marble the three batters—do not overmix.
• Bake: place the loaf pan in the preheated oven and bake until the center is just set and a toothpick inserted in the center comes out clean, about 1 hour 30 minutes. (Oven times vary; begin checking near the end of the time.)
• Cool and remove: let the cake cool in the pan on a wire rack for 15 minutes. Use the parchment overhang to lift the cake from the pan, then cool completely on the wire rack.