Decorate cookies. Prepare decorating bag with thinned royal icing. Cut small hole in tip of bag. Pipe melting snowman shape on each cookie, being careful not to go over the edge of the cookie; flood in.
Immediately sprinkle the edge of the shape with white nonpareils.
Place marshmallows and arms on cookies.
Use black rainbow chips for buttons.
Let cookies dry, at least 24 hours.