Preheat oven to 350°F. Line a 9x9x2" metal baking pan with foil, leaving a 2" overhang. Bring stout to a boil in a medium saucepan; cook until reduced to 1/2 cup, about 12 minutes. Let cool. Reserve 1/4 cup stout.
Stir 12 ounces chocolate and 1 cup butter in a medium metal bowl set over a saucepan of simmering water until melted and smooth. Whisk sugar, eggs, and vanilla in a large bowl to blend. Gradually whisk in chocolate mixture, then 1/4 cup stout from pan. Fold in flour and 1 1/4 teaspoons salt. Pour batter into prepared pan.
Bake brownies until surface begins to crack and a tester inserted into center comes out with a few moist crumbs attached, 35–40 minutes. Transfer pan to a wire rack and let cool for at least 20 minutes. Stir remaining 4 ounces chocolate in a medium metal bowl set over a saucepan of simmering water until melted and smooth. Add reserved 1/4 cup reduced stout, remaining 2 tablespoons butter, and 1/4 teaspoon salt; CBD oil, and whisk until well blended. Pour over brownies and let cool slightly before serving
