Vanilla-Chocolate Marble Slab Cake

To make this easy slab cake into a rather more decadent affair, frost it with chocolate-flavoured whipped cream or Quick Chocolate Buttercream. If you prefer a plain vanilla cake, omit the cocoa powder mixture and simply scrape all of the batter into the prepared pan, then bake as directed.

MAKES: one 13-x 9-inch cake; 18 to 24 servings

PREP TIME: 20 minutes

COOKING TIME: 45 to 50 minutes

INGREDIENTS:

• 5 ¼ cups all-purpose flour
• 5 tsp baking powder
• 1 ½ tsp fine sea salt
• 1 ½ cups unsalted butter, softened
• 3 cups granulated sugar
• 9 large eggs
• 2 Tbsp vanilla extract
• 2 cups buttermilk
• ⅔ cup unsweetened cocoa powder
• ⅓ cup plus 1 Tbsp hot water

INSTRUCTIONS:

1. Preheat the oven to 350°F. Butter a 13-x 9-inch baking pan, then line the base of the pan with parchment paper. Grease the parchment paper, then dust the parchment and the sides of the pan with flour. Set aside.
2. In a medium bowl, whisk together the flour, baking powder and salt. Set aside.
3. In a large bowl and using an electric mixer, beat the butter with the sugar until pale and fluffy, about 5 minutes. Scrape down the sides of the bowl once or twice with a spatula.
4. Beat in the eggs one at a time, beating well after each addition and scraping down the sides of the bowl as needed. Beat in the vanilla extract.
5. With the mixer running on low speed, add the flour mixture to the sugar mixture alternately with the buttermilk, making three additions of flour and two of buttermilk, and beginning and ending with the flour. Beat until well combined. Scrape half of the batter into a separate bowl.
6. In a small bowl, whisk the cocoa powder with half of the hot water until a smooth paste forms. Whisk in the remaining water until smooth. Add the cocoa mixture to one of the bowls of batter. Beat until well combined.
7. Spoon 4 large spoonfuls of the vanilla batter into the prepared pan, spacing them far enough apart that you can create a checkerboard pattern when you add the chocolate batter. Spoon the chocolate batter into the empty spaces to cover the bottom of the pan completely.
8. Repeat with a second layer, spooning the remaining vanilla batter over the chocolate and vice versa. Using the handle of a wooden spoon, swirl the batter to create a marbled effect. Tap the pan lightly on the counter a couple of times to remove any air bubbles.
9. Bake until the cake is lightly browned and a wooden skewer inserted in the centre of the cake comes out clean, 45 to 50 minutes, rotating the pan 180 degrees halfway through baking time.
10. Let the cake cool in the pan on a wire rack for 5 to 10 minutes. Run a knife around the edge of the pan to loosen the cake. Turn the cake out onto the wire rack and let cool completely. (The cake can be stored in an airtight container at room temperature for up to 3 days, or frozen for 2 months.)