Line 9 inch square or 13x9 inch pan with foil so that foil extends over sides of pan; butter foil. In large saucepan, combine sugar, margarine and evaporated milk. Bring to a boil over medium heat, stirring constantly. Boil 5 minutes, stirring constantly. Remove from heat. Add marshmallow cream and chocolate chips; blend until smooth. Stir in walnuts and vanilla. Pour into buttered, foil-lined pan. Cool to room temperature. Score fudge into 36-48 squares. Refrigerate until firm. 
Remove fudge from pan by lifting foil; remove foil from sides of fudge. Using large knife, cut through scored lines. Store in refrigerator. 
Makes about 36-48 squares.