From allrecipes.com. It works best to double the recipe and put it in a 9X13 pan.
3 cups semi-sweet chocolate chips
1 (14 oz) can sweetened condensed milk
1/4 cup butter
1 cup chopped walnuts (optional)
Place the chocolate chips, sweetened condensed milk, and butter in large microwave-safe bowl. Zap in microwave on medium until chips are melted, about 3-5 minutes, stirring once or twice during cooking. Stir in nuts, if desired. Pour into well-greased 8X8 glass baking dish. Refrigerate until set.