For the cake:

4 ounces (1 stick) unsalted butter, plus more for greasing the pan

8 ounces all-purpose flour

5 ounces sugar

5 ounces light brown sugar

2 ounces cocoa powder

1 teaspoon baking powder
1 teaspoon baking soda

½ teaspoon fine sea salt

2 large eggs

½ cup sour cream

½ cup buttermilk

1 ½ teaspoons vanilla extract

2 ounces semisweet chocolate, chopped or scant 1/3 cup semisweet chocolate chips

 

For the buttercream frosting:

5 ounces semisweet chocolate, chopped or 2/3 cup semisweet chocolate chips

6 ounces (1 ½ sticks) unsalted butter, at room temperature

¾ cup powdered sugar

½ teaspoon vanilla extract

 

Make the cake:

Preheat the oven to 375˚F. Butter a 9-inch square cake pan. Whisk together the flour, sugar, brown sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl. Whisk together the eggs, sour cream, buttermilk, and vanilla in a medium bowl. Combine the butter and chocolate in a small bowl, place the bowl over a small pan of simmering water, and heat, stirring frequently, until melted. Whisk into the egg mixture. Add the egg mixture to the flour mixture and stir until just combined. Transfer to the cake pan and spread evenly. Bake for 35 to 40 minutes, or until the edges of the cake start to shrink away from the pan and a toothpick inserted into the center comes out clean. Let the cake cool in the pan for about 10 minutes. Invert onto a cooling rack and finish cooling completely.

 

Make the buttercream frosting:

Place the chocolate into a small bowl, place the bowl over a small pan of simmering water, and heat, stirring frequently, until melted. Let cool slightly. In a mixer fitted with a paddle attachment, beat the butter on medium until light and creamy. Add the powdered sugar and vanilla and beat until blended. Add the chocolate and continue to beat until smooth, stopping the mixer once or twice to scrape down the sides of the bowl.

 

Assemble the cake:

Transfer the cake to a cake plate. Spread the frosting evenly over the top and sides of the cake, swirling decoratively. Cut into portions and serve.

 

Makes 1 9-inch square cake, serving one husband, or if you manage to hide it from the husband, 9 to 12 other people. Keeps well and stays moist for several days tightly sealed at room temperature. The recipe can be doubled and baked in a 9×13-inch rectangular cake pan—it’s a tight fit but I’ve done it.