1. Heat oven to 350°F.
2. Beat butter and sugar in large bowl until fluffy. Add egg and vanilla; beat well. Stir together flour, baking soda and salt; gradually add to butter mixture, beating until blended. Stir in mint chocolate chips. Drop by rounded teaspoons onto ungreased cookie sheet.
3. Bake 10 to 12 minutes or just until lightly browned. Cool slightly; remove from cookie sheet to wire rack. Cool completely. Makes about 36 cookies.