This recipe comes from the Williams Sonoma blog, Taste and is easy and delicious. The sour cream in both the cake and the frosting give it a wonderful texture. Be sure to have your sour cream at room temperature, especially for the frosting or the chocolate will seize up. You can make this a day ahead, refrigerate it and then serve it at room temperature.
1 1⁄4 cups (61⁄2 oz/200 g) all-purpose (plain) flour
1⁄2 cup (11⁄2 oz/45 g) unsweetened cocoa powder
1⁄2 teaspoon salt
1⁄2 teaspoon baking powder
1⁄2 teaspoon baking soda (bicarbonate of soda)
3⁄4 cup (6 oz/185 g) unsalted butter, at room temperature
1 1⁄4 cups (9 oz/280 g) firmly packed light brown sugar
3 eggs, at room temperature
2⁄3 cup (5 oz/160 g) sour cream
Sour Cream Fudge Frosting (recipe follows)
Preheat the oven to 350°F (180°C). Butter the bottoms and sides of two 9-inch (23-cm) round cake pans and line the bottoms with parchment (baking) paper. Butter the paper.
In a small bowl, sift together the flour, cocoa, salt, baking powder, and baking soda. In a large bowl, using an electric mixer on medium-high speed, beat the butter until light, about 4 minutes. Gradually add the brown sugar and beat until fluffy, about 2 minutes. Add the eggs one at a time, beating well after each addition. On low speed, add the flour mixture in 3 batches, alternately with the sour cream in 2 batches, beginning and ending with the flour mixture and mixing until incorporated.
The batter will be thick. Divide the batter evenly between the prepared pans and smooth the tops with a rubber spatula. Bake until a toothpick inserted into the centers comes out clean, about 25 minutes. Let cool in the pans on wire racks for 10 minutes. Invert onto the racks, peel off the parchment, turn right sides up, and let cool completely.
Place 1 cake layer, top side down, on a plate. Using an icing spatula, spread 2⁄3 cup (5 fl oz/150 ml) of the frosting over the top. Place the second cake layer, top side down, on top, lining up the edges. Spread the remaining frosting decoratively over the top and sides of the cake. Serve right away, or cover and store at room temperature for up to 1 day.
Sour Cream Fudge Frosting
When a cake calls for something ultrarich, creamy, and chocolaty, this is the answer. Sour cream can curdle if exposed to high heat, so be sure to let the butter-cream-chocolate mixture cool until it is just warm enough to melt the sour cream when it is added.
Makes about 2 2/3 cups (21 fl oz/660 ml)
4 tablespoons (2 oz/60 g) unsalted butter
1⁄4 cup (2 fl oz/60 ml) heavy (double) cream
10 oz (315 g) bittersweet chocolate, chopped
3⁄4 cup (6 oz/180 g) sour cream
1 cup (4 oz/125 g) confectioners’ (icing) sugar
In a heavy saucepan, combine the butter and cream over low heat and heat, stirring frequently, until the butter melts. Add the chocolate and whisk until melted and smooth, about 2 minutes. Remove the mixture from the heat and let cool to lukewarm, about 8 minutes.
Whisk in the sour cream until fully combined, then whisk in the confectioners’ sugar. Let the frosting stand until thick enough to spread, about 10 minutes.
If the frosting becomes too stiff to spread, rewarm it briefly over low heat and whisk again until smooth.
For mocha frosting: Add 1 tablespoon instant espresso powder or regular coffee powder to the butter and cream.
For tangerine-fudge frosting: Whisk in 1 tablespoon grated tangerine zest before adding the confectioners’ sugar.