Yield: approximately 12 servings
½ cup hot water
3 ounces unsweetened chocolate, chopped
1 tablespoon instant coffee powder
½ cup unsalted butter, softened
2 cups sugar
2 large eggs
1 teaspoon vanilla extract
2 cups sifted cake flour, such as Swans Down
1 teaspoon baking soda
1 cup sour cream
8 ounces semisweet chocolate, chopped
½ cup heavy whipping cream
1. Preheat oven to 350°. In a medium bowl, combine 1⁄2 cup hot water, unsweetened chocolate, and coffee. Stir until chocolate melts and coffee powder dissolves. Set aside.
2. In a large bowl, beat butter and sugar at high speed with an electric mixer until light and fluffy. Add eggs and vanilla, mixing to combine. Add melted chocolate, stirring to combine. Add flour, baking soda, and sour cream; mix just until combined. Pour batter into a 12-inch cast-iron skillet.
3. Place semisweet chocolate and cream in a microwavable bowl. Microwave on High for 1 minute. Let chocolate mixture sit for 5 minutes; whisk until smooth. Pour melted chocolate over batter, and swirl into cake with a spoon.
4. Bake until sides pull away from pan, approximately 30 minutes. Serve cake warm from skillet.