Preheat the oven to 350ºF. Line a 12-cup muffin pan with cupcake liners. Lightly spray each linger with nonstick spray– this is optional but helps prevent the cupcakes from sticking to the liners.
Make the cupcakes: Whisk the flour, cocoa powder, baking powder, and salt together. Set aside. In another bowl (or liquid measuring cup for easy pouring), combine the cream, Irish cream liqueur and vanilla.
Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar on high speed until smooth and creamy, about 1-2 minutes. Scrape down the sides and bottom of the bowl with a rubber spatula as needed. Reduce to medium-low speed and add the eggs one at a time, making sure each one is incorporating before the next.
With the mixer on low speed, alternate adding the dry ingredients and wet ingredients, ending with dry ingredients. Beat just until combined. Do not over-mix. Scrape down the sides and bottom of the bowl with a rubber spatula as needed.
Spoon the batter into the prepared liners– fill about ¾ of the way full. Bake for 20-22 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Allow the cupcakes to cool completely before frosting.
Make the frosting: With a handheld or stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until creamy, about 2 minutes. On low speed, add the powdered sugar, salt, vanilla, and Irish cream liqueur (2 Tablespoons at a time). Beat together, gradually increasing to medium-high speed and beat for 1 full minute. Add up to ¼ cup more powdered sugar if frosting is too thin. Add a pinch more of salt if frosting is too sweet.
Frost the cooled cupcakes. Cover and store leftover cupcakes in the refrigerator for up to 5 days.