How to Make It
Prepare the Brownies: Preheat oven to 350°F. Line a 9- x 13-inch baking pan with aluminum foil, allowing ends of foil to extend 2 to 3 inches beyond edges of the pan. Lightly coat foil with cooking spray.
Pour beer in a medium saucepan over medium-high; let simmer until reduced to 1/2 cup, about 15 minutes. Whisk in butter, semisweet chocolate, and unsweetened chocolate until melted and mixture is smooth. Let cool 20 minutes.
Whisk together sugar, eggs, and vanilla in a large bowl. Stir in cooled chocolate mixture until combined, and set aside.
Whisk together flour, baking powder, and salt in a medium bowl. Gradually stir flour mixture into chocolate mixture, and pour into prepared baking pan.
Bake in preheated oven until a toothpick comes out clean, 35 to 40 minutes, being careful not to over-bake. Allow brownies to cool completely on a rack, about 2 hours. (Brownies can be refrigerated overnight and frosted the next day.)
Prepare the Irish Whiskey Frosting: Beat together powdered sugar and butter with an electric mixer on low speed until well blended. Scrape down sides of bowl, and beat on medium speed until fluffy, 2 to 3 minutes. Beat in whiskey and cream, until light and fluffy. Spread frosting in an even layer on brownies, and sprinkle with sea salt. Cut brownies into 24 squares.