Heat oven to 350 degrees. Grease a 9x13-inch pan. In a large mixer bowl, combine first 5 ingredients; beat until smooth. Pour into prepared pan. Bake 25 to 30 minutes, or until top springs back when tapped lightly. Cool completely in pan. In a small bowl, combine confectioners' sugar, butter, water, mint extract and food coloring. Beat until smooth. Spread this on cake. Cover and chill. In a small saucepan over low heat, melt butter and chocolate chips. Remove from heat; stir until smooth. Cool slightly. Pour over chilled cake. Cover and chill at least 1 hour before serving. ----