Guinness Brownies with Irish Whiskey Frosting

While few treats are more appropriate for a St. Patrick's Day celebration than hearty Guinness brownies, we would encourage you to make this Guinness brownie recipe anytime you want a decadent serving of chocolate. In fact, with the dangerously delicious whiskey-infused buttercream used to freeze them, these fudge brownies are truly the perfect treat to savor at the end of an exceptionally long day.
Prepare the brownies: Preheat the oven to 350 ° F. Line a 9-inch x 13-inch baking dish with aluminum foil so that the ends of the foil protrude 2-3 inches beyond the edges of the pan. Lightly brush the foil with cooking spray.Pour the beer into a medium saucepan over medium-high heat; simmer until reduced to 1/2 cup, about 15 minutes. Whisk together the butter, semisweet chocolate, and unsweetened chocolate until they melt and the mixture is smooth. Let cool for 20 minutes.Whisk together the sugar, eggs, and vanilla in a large bowl. Add the cooled chocolate mixture until smooth and set aside.Whisk together the flour, baking powder, and salt in a medium bowl. Gradually combine the flour mixture with the chocolate mixture and pour into the prepared baking dish.Bake in the preheated oven until a toothpick comes out clean, 35 to 40 minutes, being careful not to overcook. Let the brownies cool completely on a wire rack for about 2 hours. (Brownies can be stored in the refrigerator overnight and frosted the next day.)Prepare the Irish whiskey frosting: Beat together the powdered sugar and butter in an electric mixer on low speed until smooth. Peel the sides of the bowl and beat on medium speed until fluffy, 2-3 minutes. Whisk the whisky and cream together until light and fluffy. Spread the brownies evenly with icing and sprinkle with sea salt. Cut the brownies into 24 squares.