Chocolate Fudge

12 oz. package chocolate chips
1 can sweetened condensed milk
1 cup chopped walnuts 
1 cup miniature marshmallows
Melt chocolate chips over a double boiler. Stir chips 
as they melt. When they are melted, pour in sweetened 
condensed milk, keep stirring. When mixture is smooth, 
stir in chopped walnuts. Before you are ready to pour 
fudge out, stir in marshmallows. Pour out in a buttered 
8 by 8 by 2 inch pan. Cool. Chill in the refrigerator 
for a few hours. Cut in squares and store fudge in an 
air-tight container in the refrigerator.