Beat butter and cream cheese at medium speed of an electric mixer until creamy; gradually add sugar, beating well. Add egg, beating well. Add melted chocolate, milk, and vanilla; beat well.
Combine flour and next 3 ingredients; add to butter mixture, beating well. Stir in marshmallow cream and remaining ingredients, mixing well.
Drop dough by heaping tablespoonfuls onto lightly greased cookie sheets. Bake at 325° for 13 minutes or until done. Cool 1 minute on cookie sheets; remove to wire racks and cool completely.