16 ounces stout beer (such as Sam Adams Cream Stout or Guinness)
12 ounces bittersweet chocolate, chopped
2 sticks (8 oz) butter, in pieces
5 large eggs, lightly beaten
2 1/4 cups sugar
2 teaspoons vanilla extract
1 1/4 cups cake flour
1 cup unsweetened cocoa powder
1/2 teaspoon salt
1 cup pecans, lightly toasted and chopped
For the ganache
8 ounces bittersweet chocolate, chopped
2/3 cup heavy cream
2 tablespoons butter
2-3 tablespoons Irish whiskey
Butter a 13X9-inch baking pan. Line the pan with foil or parchment paper so that there is a 2-inch overhang. Butter the foil/parchment.
Pour the stout into a medium saucepan and set it over medium-high heat. Bring the stout to a boil, reduce the heat to medium, and allow the stout to reduce down to 3/4 cup, 10-15 minutes. Allow the stout to cool slightly.
Melt the chopped chocolate with the butter in a large metal bowl placed over a saucepan of simmering water, stirring until smooth. Remove the bowl from the saucepan and gradually add the eggs, whisking constantly until the mixture thickens to a pudding-like texture. Whisk in the sugar until fully incorporated, then whisk in the vanilla.
Sift together the cake flour, cocoa, and salt into a medium bowl. Add the flour mixture to the chocolate mixture in 2 additions, alternating with 2 additions of the reduced stout, stirring until fully blended. Mix in the pecans.
Pour the batter into the prepared pan and bake until the top is set and a toothpick inserted emerges with a few moist crumbs attached, 24-30 minutes. Place the pan on a rack to cool.
Prepare the ganache: Melt the chocolate, cream, and butter in a metal bowl placed over a saucepan of simmering water, stirring until smooth. Remove from the heat and stir in 2-3 tablespoons of the whiskey, depending on personal taste. Allow the mixture to cool to lukewarm, then spread it over the brownies in the pan, smoothing the top. Chill the brownies for at least 2 hours before cutting.