Reduce the Guinness. In a small saucepan, bring the Guinness to a boil over medium-high heat and simmer until reduced to 2/3 cup, about 20-22 minutes. Set aside to cool for at least 10 minutes.

Preheat the oven to 350°F (177°C) and grease a 9-inch square baking pan or line with parchment paper, leaving overhang.

Place the butter and chopped chocolate in a large microwave-safe bowl and melt in 30-second increments until smooth.

Whisk in sugar and 1/2 cup of reduced Guinness until combined, then add eggs and vanilla extract.

Whisk in flour, salt, and optional espresso powder until the batter is thick and shiny.

Pour the batter into the prepared pan and bake for about 32-35 minutes, checking for moist crumbs with a toothpick.

Remove from the oven and let them cool completely on a wire rack before frosting or cutting into squares.

In a bowl, beat softened butter on high speed until smooth, add confectioners' sugar, then mix in 2 tablespoons of reduced Guinness, espresso powder, vanilla extract, and salt. Adjust consistency as needed.

Once cooled, spread frosting over the brownies. Cover and store leftovers at room temperature.