1. Preheat oven to 350°. Line bottom and sides of an 8-inch pan with aluminum foil, allowing 2 to 3 inches to extend over sides; lightly grease foil.
2. Microwave chocolate squares and 3/4 cup butter in a large microwave-safe bowl at HIGH 1 1/2 to 2 minutes or until melted and smooth, stirring at 30-second intervals. Whisk in 1 1/2 cups granulated and 1/2 cup brown sugars. Add eggs, 1 at a time, whisking just until blended after each addition. Whisk in 1 cup flour, vanilla, and 1/8 teaspoon salt.
3. Pour mixture into prepared pan.
4. Stir together 1/2 cup flour, 2 Tbsp. brown sugar, 2 Tbsp. granulated sugar, peanut butter, 2 Tbsp. melted butter, and 1/8 tsp. salt until blended and crumbly. Sprinkle peanut butter mixture over batter.
5. Bake at 350° for 50 to 54 minutes or until a wooden pick inserted in center comes out with a few moist crumbs. Cool completely on a wire rack (about 1 hour). Lift brownies from pan, using foil sides as handles. Gently remove foil, and cut brownies into 16 squares.