Step 1: Preheat the oven to 350°F.

Step 2: If you are using ale, pour it into a heavy saucepan and bring to a boil over high heat. Boil the ale until it is reduced by half (to 1/2 cup), about 5 minutes. Place the reduced ale in a heatproof bowl and let it cool. If you are using stout, there’s no need to boil it down.

Step 3: Coarsely chop the bittersweet chocolate into 1/2-inch pieces and place them in a separate bowl. Add 1 tablespoon of the flour and toss to coat the chocolate (this keeps the chocolate from sinking to the bottom of the batter).

Step 4: Wipe out the saucepan, then add the butter and melt it over low heat. Brush or smear a little of the butter all over the inside of a 9-by-13-inch baking pan. Refrigerate the pan to chill it for about 5 minutes, then sprinkle 1 tablespoon of flour inside the baking pan, shaking and tilting the pan to coat the bottom and sides.

Step 5: Add the unsweetened chocolate to the melted butter in the saucepan and melt it over low heat, about 4 minutes, stirring with a rubber spatula.

Step 6: Place the eggs and granulated sugar in a mixing bowl and whisk to mix. Whisk in the reduced ale or the stout and the vanilla, salt, and the melted butter and chocolate mixture. Stir in the remaining flour and the bittersweet chocolate chunks. Spoon the batter into the prepared baking pan and place it in the oven.

Step 7: Bake the brownies until the top is puffed and firm to the touch, 20 to 30 minutes. A skewer or toothpick inserted in the center will come out mostly dry—a little stickiness is OK.

Step 8: Remove the brownies from the oven and let cool in the pan to room temperature. Cut the brownies into 12 rectangles for serving and dust them with the confectioners’ sugar if desired.