1. Heat oven to 350°F.
2. Beat butter and sugar in large bowl until creamy. Add egg and vanilla; beat well. Stir together flour, cocoa, baking soda and salt; gradually add to butter mixture, beating well. Stir in mint chocolate chips. Drop by rounded teaspoons onto ungreased cookie sheet.
3. Bake 8 to 9 minutes or just until set; do not overbake. Cool slightly; remove from cookie sheet to wire rack. Cool completely. Makes about 36 cookies.