Preheat oven to 350°F. Line 2 (8-inch-square) baking pans with parchment paper, leaving a 2-inch overhang on all sides to easily lift brownies out of pans.

Place butter in a microwavable bowl, and heat on HIGH in 30-second intervals, stirring after each, until melted (about 1 minute total). Stir in chocolate until melted; stir in vanilla, and set aside.

Stir together flour, cocoa, and salt in a large bowl; set aside.

Place granulated sugar, brown sugar, eggs, and yolk in a large bowl. Beat with an electric mixer on high speed until pale and thick, about 1 1/2 to 2 minutes. Stir in melted chocolate mixture. Using a rubber spatula, gently fold in flour mixture. (Batter will be very thick.) Fold in stir-ins (below), if desired. Divide batter evenly between prepared pans.

Bake in preheated oven until a wooden pick inserted in center of each pan comes out with a few moist crumbs and tops are set and shiny, 25 to 28 minutes. Remove from oven, and cool completely in pans on wire racks, 1 hour. Use parchment handles to lift cooled brownies from pans; cut and serve.