In a large mixing bowl cream together the butter, granulated sugar and brown sugar until fluffy. Mix in the egg and vanilla extract.
Whisk together the flour, cocoa, baking soda and salt. Combine with the butter mixture and mix until well combined. Add in the chocolate chips and mallow bits (save some to add to the tops of the dough once the cookies are scooped) and mix until just incorporated.
Scoop even sized cookies onto ungreased cookie sheets and press down gently to flatten the tops. Add mallow bits to the top of the cookies. Bake cookies for 10-12 minutes in the preheated oven. Let cool completely before eating.