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1
Make the cupcakes Preheat the oven to 350 and line a 12-cup muffin pan with paper liners.
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2
In a bowl, whisk the flour with the cocoa powder, baking powder, salt and baking soda.
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3
In a large bowl, using a handheld electric mixer, beat the butter with the sugar at medium-high speed until fluffy.
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4
Beat in the eggs 1 at a time, then beat in the sour cream and vanilla until smooth.
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5
At low speed, beat in the dry ingredients.
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6
Scoop the batter into the lined muffin cups.
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7
Bake the cupcakes in the center of the oven for about 17 minutes, until springy and a toothpick inserted in the centers comes out clean.
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8
Let cool slightly in the pan, then transfer to a rack to cool completely.
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9
Meanwhile, make the frosting In a large bowl, using a handheld electric mixer, beat the butter at medium speed until smooth.
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10
Add the confectioners sugar, vanilla, salt and 2 tablespoons of the milk and beat at low speed just until combined, then beat at medium speed until smooth.
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11
Scrape half of the vanilla frosting into a medium bowl.
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12
Add the cocoa powder and the remaining 2 tablespoons of milk to the frosting in the large bowl and beat at low speed until fully incorporated.
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13
Lay a large sheet of plastic wrap on a work surface with the long side facing you.
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14
Using a small spatula, spread the vanilla frosting in a 3-inch-wide strip down the center of the plastic wrap.
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15
Spread the chocolate frosting in a 3-inch-wide strip alongside the vanilla.
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16
Using the plastic, fold the chocolate frosting over the vanilla, twisting one end of the plastic to seal.
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17
Pull the twisted end of the plastic through a pastry bag fitted with a large star tip and cut off the protruding plastic at the tip.
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18
Refrigerate the frosting until barely firm, about 15 minutes.
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19
Pipe onto the cupcakes and serve.
