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Preheat oven to 350 degrees. Line a 12-tin cupcake pan with cupcake liners.
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Combine cocoa and hot coffee together, stirring until smooth; set aside.
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Beat butter with a standing mixer or hand mixer until creamy and fluffy, about 2-3 minutes. Gradually add in sugar, beating 4-5 minutes until it is light and fluffy. Add eggs, 1 at a time, beating just until the yellow disappears.
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In a bowl, sift together flour, baking soda, baking powder, salt, and espresso powder.
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Alternately mix the coffee/cocoa mixture and flour mixture with the butter batter, beginning and ending with the flour mixture. Do not overmix.
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Add in vanilla extract, milk, and sour cream. Mix until just combined.
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Divide batter evenly between muffin cups (the batter makes 12-14).
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Bake for 17-18 minutes, or until a cake tester comes out clean. Be careful not to overbake, as chocolate cupcakes can get very dry if overbaked.
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Allow to cool.
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For the frosting, beat butter until light and fluffy. Add in cocoa powder and 2 cups of powdered sugar. Add in heavy cream, a pinch of salt (salt brings out the chocolate flavor).
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If the frosting seems too thin, add in the other cup of powdered sugar. If it seems too thick, add in more heavy cream.
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Frost cupcakes and enjoy!