1/2cup(1 stick) unsalted buttercut into 6-8 slices
In a small saucepan over medium-high heat, bring the beer to a boil.
Once boiling, reduce to medium heat and allow to simmer until reduced down to 3/4 cup, about 10 to 15 minutes. Set aside to cool completely to room temperature.
CHOCOLATE GUINNESS CUPCAKES
Preheat the oven to 350ºF. Line the wells of one 12 count cupcake pan with cupcake liners. Set aside.
In a large bowl, whisk together the granulated sugar, brown sugar, flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
In a small bowl, whisk together the eggs, vegetable oil, and 1/2 cup of the beer reduction until completely combined. Pour the wet ingredients into the dry ingredients and whisk or stir the batter until everything is combined.
Spoon batter evenly into the cupcake liners filling ½ to ⅔ of the way full (I like to use an ice cream scoop with a trigger-- one scant scoop full). Bake for 13-15 minutes or until a toothpick inserted in the center comes out clean. Allow to cool in the pan for a few minutes, then transfer to a wire cooling rack to cool completely.
CHOCOLATE GUINNESS FROSTING
In a small saucepan over medium heat, melt the unsweetened baking chocolate and the butter. Allow to cool for 15 minutes.
In a medium bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, combine cooled chocolate/butter mixture, vanilla extract, and remaining beer reduction (there should be about 4 Tablespoons left). Turn the mixer onto low and slowly add the powdered sugar. Increase mixer speed to medium and beat until completely combined, scraping down sides as needed. Add salt to taste.
ASSEMBLE THE CUPCAKES
Frost the cupcakes using a large zip top bag with the corner snipped off or a pastry bag.
Store cupcakes covered tightly at room temperature up to 4 days or in the refrigerator up to 6 days.