1. Coat an 8x8-inch pan with cooking spray.
2. Stir together the sugar, margarine and evaporated milk. Bring to a boil in a sauce pan, reduce the heat to medium-low and cook for about 3 minutes, stirring constantly.
3. Stir in the marshmallows and chocolate. Remove from the heat and stir until smooth.
4. Pour into the pan and refrigerate for about 2 hours (until firm - for a firmer consistency, leave the fudge in the freezer).
5. Cut into 36 squares and serve.