01 -
Preheat your oven to 340°F (170°C) and line a 12-cup muffin tin with paper liners. In a large bowl, beat the butter and sugar with an electric mixer until light and fluffy, about 3 minutes. Add the egg and beat until fully incorporated. In a small bowl, mix the cocoa powder, vanilla extract, and red food coloring to form a thick paste. Add this chocolate-red paste to the butter mixture and beat until evenly combined.
02 -
Add the flour and buttermilk to the batter, alternating between the two and beginning and ending with flour (about 3 additions of flour, 2 of buttermilk). Mix just until combined after each addition. In a small cup, stir together the baking soda and vinegar – it will fizz. Quickly fold this mixture into the batter. Divide the batter evenly among the prepared cupcake liners, filling each about ⅔ full.
03 -
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Remove from the oven and let cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before frosting. The cupcakes must be completely cool or the frosting will melt.
04 -
In a mixing bowl, beat the softened butter and cream cheese until smooth and well combined, about 2 minutes. Gradually add the sifted powdered sugar, beating on low speed until incorporated, then on medium-high until light and fluffy. Transfer the frosting to a piping bag fitted with a Wilton 1M tip (or your preferred decorative tip) and pipe swirls onto each cooled cupcake. If desired, decorate with sprinkles, chocolate shavings, or small red decorations.