Place oven rack in the center position. Preheat oven to 350 degrees F (175 degrees C). Spray a 9x13-inch baking dish with cooking spray.
Whisk 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 from heat.
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 beat until just combined. Add flour mixture, 2 to 3 tablespoons at a time, until just combined. Whisk in beer until batter is thin; fold in pecans.
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.
