Makes 12 cupcakes
8 tablespoons unsalted butter, cut into 4 pieces
2 ounces bittersweet chocolate, chopped
½ cup Dutch-processed cocoa (1½ ounces) (Droste recommended)
¾ cup unbleached all-purpose flour (3¾ ounces)
½ teaspoon baking soda
¾ teaspoon baking powder
2 large eggs
¾ cup sugar (5¼ ounces)
1 teaspoon vanilla extract
½ teaspoon table salt
½ cup sour cream (4 ounces)
1. Adjust oven rack to lower-middle position; heat oven to 350 degrees. Line standard-sized muffin pan (cups have ½-cup capacity) with baking-cup liners.
2. Combine butter, chocolate, and cocoa in medium heatproof bowl. Set bowl resting inside saucepan containing barely simmering water, bowl should not touch the water.
3. Heat mixture until butter and chocolate are melted and whisk until smooth and combined. Set aside to cool until just warm to the touch.
4. Whisk flour, baking soda, and baking powder in small bowl to combine.
5. Whisk eggs in second medium bowl to combine.
6. Add sugar, vanilla, and salt and then whisk until fully incorporated.
7. Add cooled chocolate mixture and whisk until combined.
8. Sift about one-third of flour mixture (using a fine mesh strainer and spoon) over chocolate mixture and whisk until combined.
9. Whisk in sour cream until combined, then sift remaining flour mixture over and whisk until batter is homogenous and thick.
10. Divide batter evenly among muffin pan cups (a ice cream scoop works well to divide evenly).
11. Bake until skewer inserted into center of cupcakes comes out clean, 18 to 20 minutes.
12. Cool cupcakes in muffin pan on wire rack until cool enough to handle, about 15 minutes. Carefully lift each cupcake from muffin pan and set on wire rack. Cool to room temperature before icing, about 30 minutes.
Vanilla Frosting
3 cups 2 tablespoons heavy cream
1 teaspoon vanilla extract
Pinch table salt
2½ sticks unsalted butter, softened
2½ cups confectioners' sugar
1. Stir the cream, vanilla, and salt together in a small bowl until the salt dissolves.
2. Beat the butter with an electric mixer on medium-high speed until smooth, 30 to 60 seconds.
3. Reduce the speed to medium-low, slowly add the confectioners' sugar, and beat until smooth, 2 to 5 minutes. Beat in the cream mixture. Increase the speed to medium-high and beat until the mixture is light and fluffy, 4 to 8 minutes.
To Frost
Mound about 2 tablespoons icing on center of each cupcake. Using small icing spatula or butter knife, spread icing to edge of cupcake, leaving slight mound in center.
To Make Ahead
The frosting can be made up to 2 days ahead and refrigerated, wrapped tightly in plastic wrap. Before using, let stand at room temperature to soften, then whisk briefly to re-fluff.