Preheat oven to 350°F. Line inside of an 8-inch square pan with foil and spray bottom only with cooking spray.
Whisk chocolate mixture with 1 ½ cups granulated and ½ cup brown sugars. Add eggs, 1 at a time, whisking just until blended after each addition. Stir in 1 cup flour, vanilla, and ⅛ teaspoon salt.
Pour mixture into prepared pan.
Stir together ½ cup flour, 2 Tbsp. brown sugar, 2 Tbsp. granulated sugar, peanut butter, 2 Tbsp. melted butter, and ⅛ tsp. salt until blended and crumbly. Sprinkle peanut butter mixture over batter.
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.