3/4 cup cocoa powder , preferably Dutch-processed
1 1/4 cups unbleached all-purpose flour (6 1/4 ounces)
1/4 teaspoon table salt
8 ounces semisweet chocolate , chopped
12 tablespoons unsalted butter (1 1/2 sticks), plus extra for baking pan
4 large eggs
1 1/2 cups granulated sugar
1 teaspoon vanilla extract
1 cup buttermilk
1/2 teaspoon baking soda
1. Adjust oven rack to middle position and heat oven to 325 degrees. Coat bottom and sides of 9 by 13-inch baking pan with 1 tablespoon butter.
2. Sift together cocoa, flour, and salt in medium bowl; set aside. Heat chocolate and butter in microwave-safe bowl covered with plastic wrap 2 minutes at 50 percent power; stir until smooth. (If not fully melted, heat 1 minute longer at 50 percent power.) Whisk together eggs, sugar, and vanilla in medium bowl.
3. Whisk chocolate into egg mixture until combined. Combine buttermilk and baking soda; whisk into chocolate mixture, then whisk in dry ingredients until batter is smooth and glossy. Pour batter into prepared pan; bake until firm in center when lightly pressed and toothpick inserted in center comes out clean, about 40 minutes. Cool on wire rack until room temperature, at least 1 hour; serve, or ice with frosting, if desired.
To Make Ahead
After the cake(s) has cooled, it can be wrapped tightly in plastic wrap and kept at room temperature for up to 1 day. Or, wrapped tightly in plastic wrap, then foil, the cake(s) can be frozen for up to 1 month. Defrost at room temperature before unwrapping and frosting.
Recipe from Cook's Illustrated E-mail Newsletter