Chocolate Marble Cake

Instructions

1. Preheat the oven to 325°F (160°C) and grease a 9 × 13-inch (23 × 32 cm) baking pan.
2. Make the chocolate cake portion. Melt the butter and chocolate together in a medium saucepan over medium-low heat, stirring constantly with a spatula until smooth. Add the brown sugar and water, stirring together until smooth. Transfer to a large mixing bowl and cool for 5 minutes. Whisk in the sour cream by hand until smooth. Whisk in the eggs one at a time until blended in. Sift in the flour, cocoa, baking powder, and salt, and whisk just until smooth. Set aside.
3. Make the vanilla cake portion. Sift the flour, sugar, baking powder, and salt into a large mixing bowl or into the bowl of a stand mixer fitted with the paddle attachment. Using beaters or the mixer on medium-low speed, cut in the butter until the flour has an even, crumbly texture. Add the buttermilk (or kefir), egg whites, and vanilla to the flour, mixing at first on low to combine and then for a minute on medium speed until the batter becomes fluffy.
4. Swirl the two cakes in the pan. Ladle half of the chocolate batter into the prepared pan, leaving some space in between the spoonfuls (the batter will spread a touch; that’s OK). Dollop half of the vanilla batter in between the gaps in the chocolate batter. Repeat with the remaining chocolate batter, ladling it over the vanilla dollops, and conversely dollop the remaining vanilla batter over the chocolate. Use a skewer or paring knife to swirl the two cake batters together, but not so much that you lose the distinction between them.

NOTE: While vanilla cake batter will be a bit thicker than the chocolate batter, they swirl and bake up evenly. Use a random swirling motion to create small and bigger circular swirls, so they don’t look like a pattern once the cake is baked and sliced.

5. Bake for 40 to 60 minutes, until a tester inserted in the center of the cake, comes out clean. Cool completely in its pan on a rack.

NOTE: In a metal pan, the cake bakes in 40 to 50 minutes. In a ceramic pan, it takes 50 to 60 minutes.

6. Make the marble glaze. Place the dark chocolate and 6 Tbsp (90 g) of the butter in a metal bowl. Set over a pot filled with an inch of barely simmering water and stir gently until melted. Set aside. Place the white chocolate and remaining 6 Tbsp (90 g) of butter in a separate bowl and melt over the water bath while stirring gently and set aside. Heat the condensed milk over medium heat to about 115°F (46°C) while stirring. Measure ⅔ cup (150 mL) into each bowl of chocolate and whisk until smooth. Use immediately.

NOTE: The white chocolate and butter, once melted, may take on a grainy appearance. Once you add the condensed milk and whisk, the ganache comes together smoothly. Heating the condensed milk makes the finished glazes more fluid and easier to swirl together.

7. Glaze the cake. Ladle or pour some of the dark chocolate glazes onto the cake, letting it flow around the cake. Do the same with the white chocolate. Repeat with the remaining dark and white chocolate glazes and use a skewer or paring knife to swirl the glazes together. Chill the cake for at least 2 hours to set the glaze, then serve at room temperature.

The cake will keep, with the cut portioned covered, in the fridge for up to 3 days.