1.Pour the Guinness into a saucepan over medium-high heat until it bowls, then turn the head down to medium and simmer for 22 minutes. It should be reduced to about ⅔ cup. Set it aside for 10 to 15 minutes.
2.Prepare a 9” square pan and grease and line it with parchment paper, leaving some overhang. Preheat your oven to 350°F.
3.Add chopped chocolate and 12 tablespoons butter into a big microwave-safe bowl. Melt it in the microwave in 30-second intervals, stirring after each round.
4.Whisk in sugar and 1/2 a cup of reduced Guinness. Blend until mixed. Whisk in 1 teaspoon vanilla extract and eggs. Add 1/2 teaspoon salt, flour, and 1/3 teaspoon espresso powder. Mix until combined.
5.Pour this mixture evenly into your pan. Bake it for about 32 minutes. Poke it in the center with a toothpick. Your brownies are ready if the toothpick comes out mostly clean with just a few wet crumbs. Let the brownies sit on a wire rack until they are completely cool.
6.Using the bowl of an electric mixer with whisk attachment. Add 4 tablespoons butter. Mix on high until smooth, about 2 to 3 minutes. Add in powdered sugar. Start mixing on low (about 30 seconds), then crank it up to high until mixed.
8.Pour in the rest of the reduced Guinness, 1/4 teaspoon vanilla extract, 1/2 teaspoon espresso powder, and 1/8 teaspoon salt. Adjust taste or texture to your liking. If the frosting feels too thick, add some milk.
9.Spread this frosting on your cooled brownies, cut and enjoy!