Preheat the oven to 375 degrees F (190 degrees C). Grease cookie sheets.
In a large bowl, cream together butter, brown sugar and white sugar until smooth. Beat in the eggs one at a time, then stir in the vanilla. Combine the flour, cocoa, baking soda and salt; stir into the creamed mixture. Mix in the chocolate chips and marshmallows. Drop by rounded spoonfuls onto the prepared cookie sheets about 2 inches apart.
Bake for 8 to 10 minutes in the preheated oven. Allow cookies to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.