Mix together 1/2 cup softened butter, sugar, egg, 1 teaspoon vanilla, cocoa, flour, soda, salt, and milk.
Drop by small teaspoon onto ungreased cookie sheet. Bake at 375 degrees for 5 minutes, remove from oven, and top with 1/2 marshmellow. Return to oven for 1 minute. Remove from oven and cool.
Icing: Mix thoroughly together 1/3 cup softened butter, melted chocolate, confectioners' sugar, and vanilla. Beat until smooth and of spreading consistency (add more milk to thin or more confectioners' sugar to thicken). Spread on cookies. Dip into chopped nuts, if desired.