Coat an 8x8 inch pan with cooking spray.
In medium sauce pan, stir together sugar, evaporated milk and margarine; bring to a boil over high heat. Reduce heat to medium-low and cook, stirring constantly, about 3 minutes.
Stir in chocolate and marshmallows; remove pan from heat and stir until smooth.
Pour mixture into prepared pan and refrigerate until firm, about 2 hours.
Cut into 36 1 1/3-inch squares and serve.
Serving size is one piece.
*Store fudge in refrigerator or freeze for a firmer consistency.