In a small saucepan, bring the Guinness to a boil over medium-high heat. Once boiling, reduce to medium heat and allow to simmer until reduced down to 2/3 cup (160g/ml), about 12–15 minutes. Set 1/2 cup (120g/ml) aside to cool for 10–15 minutes before using in the cupcake batter. While it’s warm, mix the rest of the reduced Guinness (about 2–2.5 Tablespoons) with the 2 teaspoons of espresso powder used in the frosting. Set this mixture aside for step 5. You can do this step 1 day ahead of time if needed. Refrigerate overnight. Bring reduced beer to room temperature before using in both the cupcake batter and frosting.

Preheat the oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners. Line a second pan with 4 liners—this recipe makes about 16 cupcakes. Set aside.

Whisk the flour, cocoa powder, espresso powder, baking soda, and salt together in a large bowl until thoroughly combined. Set aside. In a medium bowl, whisk the oil, brown sugar, eggs, sour cream, and vanilla together until completely smooth. Pour the wet ingredients into the dry ingredients, add the 1/2 cup (120ml) reduced Guinness, then whisk until *just* combined. Do not over-mix. The batter will be on the thinner side.

Pour or spoon the batter into the liners. Fill each 2/3 full to avoid spilling over the sides or sinking. Bake for 19–22 minutes, or until a toothpick inserted in the center comes out clean. Don’t over-bake, which will dry out the cupcakes. Begin checking for doneness at 19 minutes. For around 36 mini cupcakes, bake for about 11–13 minutes, same oven temperature. Allow the cupcakes to cool completely before frosting.

Have the Guinness/espresso powder mixture from step 1 handy. In a large bowl using a handheld or stand mixer fitted with a whisk attachment, beat the butter on high speed until completely smooth and creamy, about 2–3 minutes. Add the confectioners’ sugar, cocoa powder, reduced Guinness/espresso powder mixture, milk, and vanilla extract. Beat on low for 30 seconds then increase to high speed and beat until combined and creamy. Taste. Add a pinch of salt if desired. (I always add 1/8 teaspoon.) If you want the frosting a little thicker, add 1/4 cup (30g) of confectioners’ sugar.

Frost cooled cupcakes. I piped a basic tall swirl using Ateco 849 piping tip.

Leftover cupcakes keep well covered tightly in the refrigerator for 3 days. I recommend a cupcake carrier for storing and transporting decorated cupcakes.