Bring the Guinness to a boil in a small saucepan, then reduce heat and simmer until it reduces down to about 2/3 cup. Allow it to cool.

Preheat your oven to 350°F (177°C) and prepare a 9-inch square baking pan by greasing it with butter.

In a mixing bowl, melt the butter and coarsely chopped semi-sweet chocolate. Stir until well combined and silky.

Whisk in the granulated sugar and the reduced Guinness into the chocolate mixture.

Add the eggs one at a time along with the vanilla extract, whisking until smooth.

Gently mix in the all-purpose flour, salt, and optional espresso powder until no dry flour remains.

Pour the brownie batter into the prepared pan and spread it evenly.

Bake for about 32-35 minutes or until a toothpick inserted into the center comes out with moist crumbs.

Allow the brownies to cool completely in the pan.

While cooling, prepare the frosting by beating the unsalted butter until creamy.

Gradually mix in the confectioners’ sugar, then add the reduced Guinness, espresso powder, vanilla extract, and salt until well blended.

Frost the cooled brownies generously and cut into squares.