In a medium bowl, mix together the butter and sugar until light and fluffy. Stir in the eggs and vanilla. Sift together the flour, cocoa, baking soda and salt, stir into the creamed mixture. Finally, stir in the M&M's (TM).
Drop by rounded tablespoonfuls onto an unprepared cookie sheet. Bake for 8 to 10 minutes in the preheated oven. Be careful not to overbake, the cookies will be puffed up when they are done, but flatten when cool. Allow cookies to cool on the baking sheet for 1 minute before removing to wire racks to cool completely.