-- Softened butter and all-purpose flour for the pan
2 ounces unsweetened chocolate, coarsely chopped
2 ounces bittersweet chocolate, coarsely chopped
1 3/4 cups unbleached all-purpose flour
3/4 teaspoon baking soda
3/4 teaspoon baking powder
1/4 teaspoon salt
1/2 pound (2 sticks) salted butter, at room temperature
1 1/4 cups sugar
3 large eggs, at room temperature
1 cup sour cream
1 tablespoon pure vanilla extract
1/2 cup miniature semisweet chocolate chips
-- Grated zest of 2 blood oranges
1/2 cup fresh blood orange juice
1/4 cup sugar
Position an oven rack in the center of the oven and preheat the oven to 350F. Lightly butter and flour a 9-inch fluted or plain tube pan and tap out the excess flour.
To make the cake: In a microwave oven on Medium (50%) power, in a microwave-safe bowl, heat the unsweetened and bittersweet chocolates together for 30-second intervals, stirring after each interval, until melted, about 2 minutes. Let cool until tepid.
In a medium bowl, whisk together the flour, baking soda, baking powder, and salt. In a large bowl, beat the butter and sugar with an electric mixer set on high speed until light and fluffy, about 3 minutes. One at a time, beat in the eggs, scraping down the sides of the bowl after each addition. With the mixer on low speed, gradually add the flour mixture, and then the sour cream and vanilla.
Transfer half of the batter to a medium bowl. Add the tepid chocolate mixture and chocolate chips and stir well. Add the orange zest to the plain batter. Spoon the batters alternately into the prepared pan. Using a table knife, swirl them together to create a marbled effect.
Bake until a long bamboo skewer inserted in the center of the cake comes out clean, about 50 minutes. Let cool in the pan on a wire cooling rack for 10 minutes.
Meanwhile, make the syrup: In a small saucepan, bring the orange juice and sugar to a boil over medium heat, stirring often to dissolve the sugar. Strain into a glass measuring cup.
Invert the cake onto the cooling rack and unmold. Place the rack on a plate. Slowly drizzle the orange syrup over the cake. Using a dessert spoon, retrieve the syrup on the plate and drizzle it back over the cake. Repeat until the cake absorbs all of the syrup. Let cool completely on the rack.
Recipe reprinted with permission from Kathleen King’s Baking for Friends (Tate’s Bake Shop, Sept. 2012).