BEAT butter and sugar in large mixer bowl until creamy. Beat in egg and vanilla extract. Gradually beat in flour. Stir in 1 cup mini morsels. Press onto bottom of ungreased 13 x 9-inch baking pan.
BAKE for 30 to 33 minutes or just until top is light brown. Immediately sprinkle with remaining morsels. Let stand for 5 minutes or until morsels are shiny; spread evenly. Cool completely in pan on wire rack. Cut into squares.