1 Cup stout beer (such as Guinness)
16 oz semisweet or bittersweet chocolate, coarsely chopped, divided
1 Cup plus 2 Tbsp (2-1/4 sticks) unsalted butter
1-1/2 Cups sugar
3 large eggs
2-1/2 tsp Peppermint Extract
3/4 Cup all-purpose flour
1-1/2 tsp Kosher Salt, divided
Preheat oven to 350°F. Line a 9x9x2″ metal baking pan with foil, leaving a 2″ overhang around the sides. Bring stout to a boil in a medium saucepan on the stove; cook until reduced to 1/2 Cup, about 12 min. Allow to cool at least 30 min. Reserve 1/4 Cup of the stout for later in the recipe.
Stir 12 oz of the chocolate and 1 Cup of the butter in a medium metal bowl that has been set over a saucepan of simmering water. Heat butter and chocolate until mixture is melted and smooth. Whisk sugar, eggs, and peppermint in a large bowl to blend. Gradually whisk in chocolate mixture, then 1/4 Cup of the stout. Fold in flour and 1-1/4 tsp salt. Stir gently with a wooden spoon until batter is completely combined. Pour brownie batter into prepared pan.
Bake brownies in the preheated oven until surface begins to crack, and a tester (toothpick or knife blade) inserted into the center comes out with barely a few moist crumbs attached, approximately 35–40 min. Remove pan from oven and transfer to a wire cooling rack and let cool.
Melt the remaining 4 oz of chocolate in a medium metal bowl that has been set over a saucepan of simmering water. Stir chocolate until melted and smooth. Add to the melted chocolate the reserved 1/4 Cup of reduced stout, remaining 2 Tbsp butter, and 1/4 tsp salt; whisk until well blended.
Pour warm melted-chocolate stout glaze over the brownies. Allow glazed brownies to stand at room temperature until glaze is set, about 40 min.
DO AHEAD: Brownies can be made up to 8 hours ahead. Cover and let stand at room temperature.
To serve: Using foil overhang, lift brownies from pan in one big “chunk”; cut into serving-size squares.