Preheat oven to 350. In large mixer bowl, beat butter and sugar until creamy. Beat in egg and vanilla. Gradually beat in flour. Stir in 1 cup mini chocolate chips. Spread batter into ungreased 13x9" pan. Bake 30 to 33 minutes just until top begins to brown. Immediately sprinkle remaining 1 cup mini chocolate chips over top. Let stand 5 minutes or until chips become shiny and soft; spread chocolate over top. Cool 10 minutes. While still warm cut into 1 1/4 inch squares. Cool completely. Makes about 3 dozen.