Melt butter with sugar in a saucepan over medium-low heat, stirring to combine. Remove from heat, and pour into a mixing bowl. With an electric mixer on medium-low speed, beat until mixture is cooled, 4 to 5 minutes. Add eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed. Add vanilla, then cocoa mixture, and beat until combined. Reduce speed to low. Add flour mixture in two batches, alternating with the sour cream, and beating until just combined after each.
Divide batter evenly among lined cups, filling each three- quarters full. Bake, rotating tins halfway through, until a cake tester inserted in centers comes out clean, about 20 minutes. Transfer tins to wire racks to cool 15 minutes; turn out cupcakes onto racks and let cool completely. Cupcakes can be stored overnight at room temperature, or frozen up to 2 months, in airtight containers.
Let cool completely before frosting.
from Martha Stewart's Cupcakes book
makes about 8 cups
1 1/2 cups plus 2 tablespoons sugar
2/3 cup water
2 tablespoons light corn syrup
6 large egg whites, room temperature
Combine 1 1/2 cups sugar with the water and corn syrup in a small saucepan; clip a candy thermometer to side of pan. Bring to a boil over medium heat, stirring occasionally, until sugar dissolves. Continue boiling, without stirring, until syrup reaches 230 degrees.
Meanwhile, in the bowl of a standing electric mixer fitted with the whisk attachment, whisk egg whites on medium-high speed until soft peaks form. With mixer running, add remaining 2 tablespoons sugar, beating to combine.
As soon as sugar syrup reaches 230 degrees, remove from heat. With mixer on medium-low speed, pour syrup down side of bowl in a slow, steady stream. Raise speed to medium-high; whisk until mixture is completely cool (test by touching the bottom of the bowl) and stiff (but not dry) peaks form, about 7 minutes. Use immediately.