Preheat oven to 350 F. Line cupcake pan with paper liners.
Cream butter and sugar till very light and fluffy.
Add eggs one at a time, incorporating after each.
Combine flour, baking powder, and salt in a small bowl.
Alternate adding the dry ingredients and the beer until both are fully incorporated.
Fill cupcake liners most of the way to the top.
Bake for 20-25 minutes.
Cool in the pan for 5 minutes, then move to a wire rack to cool completely.
Frosting
Cream butter till light and fluffy.
Add beer and creme de cassis till well combined.
Add powdered sugar.
If the icing is too thick or stiff, add a bit more beer. If it's too thin, add more powdered sugar.
Once frosting is the desired consistency, top cooled cupcakes and serve!
Notes
You can pipe frosting using a Ziploc bag and cutting the corner off the bag. Only fill the bag about halfway with frosting, and fill the bag before cutting off the corner.