Use the recipe with the chocolate chips and make as follows: I never have trouble with it.

MICROWAVE FANTASY FUDGE

3 cups sugar
3/4 cup (1 1/2 sticks) butter or margarine
1 small can (5 oz.) evaporated milk (about 2/3 cup)
1 1/2 pkg. (12 squares) Baker's semi-sweet baking chocolate, chopped
1 jar (7 oz.) Jet-Puffed marshmallow creme
1 cup chopped Planters walnuts
1 tsp. vanilla

To prepare the fudge in the microwave, place butter in 4-quart microwaveable bowl. Microwave on HIGH 1 minute or until melted.

Add sugar and milk; mix well. Microwave 5 minutes or until mixture begins to boil, stirring after 3 minutes. Stir well, scraping down side of bowl.

Microwave 5 1/2 minutes, stirring after 3 minutes. Let stand 2 minutes.

Add chocolate and marshmallow creme; stir until completely melted. Add walnuts and vanilla; mix well.

Pour immediately into prepared pan; spread to evenly cover bottom of pan. Cool at room temperature at least 4 hours; cut into 1-inch squares. Store in tightly covered container at room temperature.

Good luck.

Source: Kraft