To Make Cookies: Cream 6 T butter, 1 cup sugar and egg in bowl.
In a separate bowl, mix together 2 cups flour, 5 T cocoa, 1 1/2 t soda, and 1 t salt. Add to butter mixture alternately with 1 cup milk and 1 teaspoon vanilla, mixing well after each addition.
Drop by teaspoonfuls onto cookie sheets. Bake for 10 minutes. Remove to wire rack to cook completely.
To Make Filling: Cream 3/4 cup margarine and 3/4 cup confectioners' sugar in bowl until light and fluffy. Blend in 1/4 teaspoon vanilla, 1 cup marshmallow creme and 1 tablespoon milk. Spread on half the cooled cookies; top with remaining cookies.