1. Line 13x9x2-inch pan with foil, extending foil over edges of pan; butter foil lightly.
2. Combine sugar, butter and evaporated milk in heavy 3-quart saucepan. Cook over medium heat, stirring constantly, until mixture comes to full rolling boil. Continue boiling, stirring constantly, until mixture reaches 234°F on candy thermometer or until syrup, when dropped into very cold water, forms a soft ball which flattens when removed from water. (Bulb of candy thermometer should not rest on bottom of saucepan.)
3. Remove from heat; immediately stir in mint chocolate chips until melted. Stir in marshmallow creme and vanilla until mixture is smooth and well blended. Pour mixture into prepared pan. Cool completely. Use foil to lift fudge from pan; peel off foil. Cut into squares. About 8 dozen pieces.
NOTE: For best results, do not double this recipe.