Melt the chocolates and the butter in a double boiler, stirring frequently.
Remove from heat.
Stir in nuts and marshmallows.
Pour into a 7" x 9" buttered pan (glass preferred).
When cool, cut into 1"-2" squares.
Notes
Alternate TechniqueI always melt the chocolate in the microwave, but you have to be super careful as it doesn't look like it's melted and it is easy to go from melted to burned. For this recipe, I microwave it for about 2 minutes and then check it. If it is not melted, I microwave it 30 seconds at a time until it is almost melted.Then I take it out and stir it to finish the melting process because a microwave does not heat the ingredients the same throughout the dish, and some of it is very hot while the rest is not. When stirred, it melts the rest.