Melt chocolate baking bars separately in small bowls in the microwave according to package directions. (myrecipes.com)
Melt 1/2 cup (1 stick) butter in small bowl in a microwave or in a small saucepan over medium heat and let cool. (bonappetit.com)
I have to laugh though, I used a too small bowl in the microwave and all the marshmallows expanded and dripped all over my microwave.. (cravingsofalunatic.com)