Melt chocolate in the top of a double boiler. Let cool.
In a mixing bowl, combine flour, baking soda and salt. In a separate bowl, cream butter and sugar together. Blend eggs into the butter mixture one at a time. Stir the vanilla, flour mixture, water and chocolate into the butter and eggs; beat well. Stir in marshmallows. Fill paper-lined cupcake pans half full.
Bake in a preheated 350 degrees F (175 degrees C) oven for 20 minutes. Cool on a wire rack before serving.
