Preheat the oven to 350 degrees and line 24 cupcake tins.
In a large bowl, combine the Guinness, oil, milk, vanilla, and eggs.
Stir in the sour cream.
In a separate bowl, combine the cocoa powder, sugar, flour, and baking soda.
Add the dry ingredients to the wet ingredients.
Divide the batter between the 24 cups, filling about 3/4 full. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Remove to a wire rack and cool completely.
For the buttercream, beat the butter until smooth and creamy. Slowly add in the powdered sugar until combined. Stir in the vanilla and heavy cream, until the icing is smooth.
Decorate the cupcakes with the buttercream and some fun sprinkles!
