1Heat oven to 350°F (325°F for dark or nonstick pan). Grease bottom only of (8- or 9-inch) square pan.
2In medium bowl, stir together cookie mix, butter, peanut butter, vanilla and egg with spoon until soft dough forms (dough will be crumbly). Pat dough into pan. Bake 18 to 22 minutes or until edges are dry and golden brown. Run knife around inside edge of pan. Cool on wire rack about 1 hour.
3Spread frosting over bars. Cut into squares, 4 rows by 4 rows.