MAKE cookie dough as directed on package, adding cocoa.
DROP dough with by tablespoonfuls about 2 inches apart on ungreased cookie sheet. Flatten each cookie to ¼-inch with a greased bottomed glass.
BAKE at 375°F for 7 to 9 minutes. Immediately top each cookie with 1 marshmallow. Return to oven; bake 2 minutes longer until marshmallow is soft. Place cookies on cooling racks to cool completely.