1. In a small saucepan, bring the Guinness to a boil over medium-high heat. Once boiling, reduce to medium heat and simmer until reduced to 2/3 cup (160 g/ml), about 12 to 15 minutes. Set aside to cool for at least 10 minutes. You will use 1/2 cup (120 g/ml) in the brownies and the rest in the frosting.

2. Preheat the oven to 350°F (177°C). Grease a 9-inch square baking pan, or line it with aluminum foil or parchment paper, leaving an overhang on the sides to lift the finished brownies out. Set aside.

3. Place the butter and chopped chocolate in a large microwave-safe bowl. Melt in 30-second increments, whisking after each, until completely smooth. Whisk in the sugar and 1/2 cup (120 g/ml) of reduced Guinness until completely combined. Whisk in the eggs and vanilla extract. Finally, whisk in the flour, salt, and espresso powder (if using). The batter will be thick and shiny. Pour or spread it evenly into the prepared pan.

4. Bake for 32 minutes, then test the brownies with a toothpick inserted into the center of the pan. If it comes out coated with wet batter, the brownies need more time. If there are only a few moist crumbs, they are done. Keep checking every 2 minutes until you have moist crumbs. The source’s brownies take about 35 minutes.

5. Remove from the oven and place on a cooling rack. Cool completely in the pan before frosting or cutting.

6. Make the frosting: in a large bowl, using a handheld or stand mixer fitted with a whisk attachment, beat the butter on high speed until completely smooth and creamy, about 2 to 3 minutes. Add the confectioners’ sugar, beating on low at first and then increasing to high speed. Once creamy and combined, beat in 2 tablespoons of reduced Guinness, the espresso powder, vanilla extract, and salt. Taste, and add the remaining reduced Guinness if needed. If you want the frosting a little thinner, beat in a splash of milk.

7. When the brownies are cool, lift them out by gripping the parchment overhangs and transfer them to a cutting board. Spread the frosting over the cooled brownies. Use a sharp knife to cut into squares.

8. Cover and store leftover brownies at room temperature for up to 1 week.