Microwave butter in a large microwave-safe bowl at MEDIUM HIGH (70%) for 45 to 60 seconds or until melted. Stir together brown sugar and peanut butter. Add eggs, next 4 ingredients, and 1/2 cup peanuts. Spoon batter into a lightly greased (with cooking spray) 8-inch square microwave-safe baking dish. Microwave at MEDIUM-HIGH (70%) for 4 minutes. Microwave at HIGH (100%) for 3 to 6 minutes or until top is almost dry. Remove from microwave, and immediately top with chopped chocolate. Let stand 5 minutes; spread melted chocolate evenly over top, and sprinkle with remaining peanuts. Cool 15 minutes. Cut into squares.