Alcohol does not 100% cook out of foods. A trace remains in case that's an issue for your guests.
The flavors of the Guinness and the cocoa are simply wonderful. If the stout is not suitable, replace that quantity with strong room-temperature coffee.
Place oven rack in the center position.
Preheat oven to 350 degrees F (175 degrees C).
Coat a 9x13-inch baking dish with butter. Be generous here.
Sift flour, cocoa powder, and salt together in a bowl.
Melt bittersweet chocolate, white chocolate chips, and butter in the top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching. Remove the bowl from the pot when the chocolate is just melted. Place on the counter.
Beat white sugar and eggs together in a large bowl with an electric mixer until light and fluffy, about 3 minutes.
Slowly add melted chocolate mixture and mix until just combined.
Mix in the stout.
Add flour mixture and pecans, if using, and until the flour has been absorbed then stop mixing.
Pour batter into the prepared baking dish; sprinkle semisweet chocolate chips over batter.
Bake in the preheated oven until a toothpick inserted in the center of the brownies comes out with moist crumbs, 25 to 30 minutes.
Cool to room temperature; dust top with confectioners' sugar.
If you dust the sugar on too soon, it'll be absorbed by the brownies. It isn't bad; it has just melted. Tastes great looks less great.
If you can stand it, wait at least half an hour, but no one will think less of you if you can't.