Yield: Two 9-inch round, 2-inch tall cake layers, and, in theory, 22 to 24 cupcakes, two 8-inch squares or a 9×13 single-layer cake
4 cups plus 2 tablespoons cake flour (not self-rising)
2 teaspoons baking powder
1 1/2 teaspoons baking soda
1 teaspoon salt
2 sticks (1 cup) unsalted butter, softened
2 cups sugar
2 teaspoons pure vanilla extract
4 large eggs, at room temperature
2 cups buttermilk, well-shaken
Preheat oven to 350°F. Butter two 9-inch round cake pans and line with circles of parchment paper, then butter parchment. (Alternately, you can use a cooking spray, either with just butter or butter and flour to speed this process up.)
Sift together flour, baking powder, baking soda, and salt in a medium bowl. In a large mixing bowl, beat butter and sugar in a large bowl with an electric mixer at medium speed until pale and fluffy, then beat in vanilla. Add eggs 1 at a time, beating well and scraping down the bowl after each addition. At low speed, beat in buttermilk until just combined (mixture will look curdled). Add flour mixture in three batches, mixing until each addition is just incorporated.
Spread batter evenly in cake pan, then rap pan on counter several times to eliminate air bubbles. Bake until golden and a wooden pick inserted in center of cake comes out clean, 35 to 40 minutes. Cool in pan on a rack 10 minutes, then run a knife around edge of pan. Invert onto rack and discard parchment, then cool completely, about 1 hour.
Chocolate Sour Cream Frosting
Cooking note: Be sure that your sour cream is at room temperature before you make the frosting. Using cold sour cream will result in the chocolate seizing a little, leaving little bits (like chocolate chips!) throughout the frosting.
Makes 5 cups of frosting, or enough to frost and fill a two layer 9-inch cake
15 ounces semisweet or bittersweet chocolate, chopped
2 1/4 cups sour cream, at room temperature
1/4 to 1/2 cup light corn syrup
3/4 teaspoon vanilla extract
Melt the chocolate in the top of a double-boiler or in a heatproof bowl over simmering water. Stir until the chocolate is melted. (Alternately, you can melt the chocolate in a microwave for 30 seconds, stirring well, and then heating in 15 second increments, stirring between each, until the chocolate is melted.) Remove from heat and let chocolate cool until tepid.
Whisk together the sour cream, 1/4 cup of the corn syrup and vanilla extract until combined. Add the tepid chocolate slowly and stir quickly until the mixture is uniform. Taste for sweetness, and if needed, add additional corn syrup in one tablespoon increments until desired level of sweetness is achieved.
Let cool in the refrigerator until the frosting is a spreadable consistency. This should not take more than 30 minutes. Should the frosting become too thick or stiff, just leave it out until it softens again. However, I found that the frosting didn't seem to thicken up very well at all and was more runny than I expected.