Preheat oven to 350°F. Line a 12-cup muffin pan with paper liners.
Whisk flour, cocoa, baking soda, baking powder, and salt in a bowl. Set aside.
Warm stout and butter in a saucepan over medium heat until melted. Remove from heat.
Whisk in both sugars until dissolved. Let the mixture cool for 5 minutes.
Add sour cream, eggs, and vanilla to the pan. Whisk until smooth and glossy.
Fold dry ingredients into the wet mixture just until combined. Do not overmix.
Divide batter among liners, filling each about two-thirds full. Bake 18–22 minutes.
Cool cupcakes in pan 5 minutes, then transfer to a rack to cool completely.
Beat cream cheese and softened butter until creamy. Add powdered sugar, vanilla, and salt; whip until fluffy.
Stream in heavy cream to reach spreadable consistency. Mix in Irish cream if using.
Spread or pipe frosting on cooled cupcakes. Serve immediately or chill to set.
Notes
Try stirring 1 tsp espresso powder into the batter to deepen the chocolate flavor. For a non-alcoholic version, replace the stout with strong brewed coffee. Store frosted cupcakes covered in the fridge for up to 3 days; bring to room temperature before serving.This recipe is an original creation inspired by classic Guinness Chocolate Cupcakes Recipe flavors. All ingredient ratios and instructions are independently developed.