First, make the cupcakes. Preheat oven to 350 degrees Fahrenheit. Line a one-dozen cupcake pan with cupcake liners (or spray with nonstick cooking spray.)
In a medium bowl, combine sugar, beer, melted butter, eggs, sour cream and vanilla bean paste. Set aside.
In another medium bowl, whisk together flour, cocoa powder, baking soda and salt. Add dry ingredients to butter mixture, stir to combine.
Using a ¼ cup measure or cupcake scoop, spoon batter into prepared cupcake pan. Bake in preheated oven for 20-25 minutes, or until a toothpick inserted into the center of the cupcakes comes out clean. Place on wire rack to cool.
While cupcakes are cooling, make the frosting. In a stand mixer fitted with the paddle attachment, combine butter, powdered sugar, Irish cream (or milk and mint extract) and vanilla bean paste. Mix on high speed for 2-3 minutes, or until ingredients are combined and buttercream is nice and fluffy.
Remove cupcakes from cupcake pan. Finish cooling if they’re not completely cool already. Using a piping bag (or large resealable bag) fitted with a large star frosting tip, frost cupcakes starting in a circle on the outer edge and working your way in.
Store in the refrigerator for up to three days if not serving immediately.