Preheat the oven to 375 degrees F. Line a 9 x 13-inch baking pan with nonstick foil.
In a medium bowl, whisk together flour, cocoa powder, and salt until evenly combined. Set aside.
Melt butter, bittersweet chocolate, and white chocolate chips in a double-boiler over very low heat, stirring constantly until melted. Remove from heat.
In a large mixing bowl, beat eggs and sugar on high speed until light and fluffy, about 3 minutes. Add melted chocolate mixture, beating until combined.
Beat reserved flour mixture into melted chocolate mixture. Whisk in Guinness stout beer. The batter will seem a bit thin. Drop semisweet chocolate chips evenly on top of batter (some will sink in).
Pour into prepared baking pan. Bake 25 to 30 minutes on center rack in the oven, until a toothpick inserted in the center comes out almost clean.
Let brownies cool, uncovered, to room temperature. Dust with confectioners' sugar before serving.
Note: The Guinness should be at room temperature. This recipe uses a little less than a standard 12-ounce bottle of Guinness stout beer. Do not include foam in the measurement. Either spoon off the foam or let it rest until the foam subsides.
Yield: 36 to 48 brownies, depending on cut size.