Preheat the oven to 350f and line two baking sheets with parchment paper
Beat the butter and two thirds of the sugar until light and fluffy which will take around 3-4 minutes.
Add the egg and beat until combined.
In a separate bowl mix together the dry ingredients of flour, cocoa powder, dry milk, baking soda and salt.
Gradually add the dry ingredients to the butter mixture mixing at low speed then add in the vanilla extract.
In a small bowl mix together the remaining one third of sugar with the cinnamon.
Take spoonfuls of the cookie dough – approximately one inch in size – roll into a ball then roll in the cinnamon sugar before placing onto the lined baking sheet
Leave at least two inches between each cookie as they will spread as they bake.
Place in the oven to bake for around 8-10 minutes.
Remove from the oven, turn the oven to broil.
Whilst the cookies are still soft, press a halved marshmallow onto the top of each cookie, cut side down.
Please under the broiler at least 5-6 inches from the heat and witch carefully until the marshmallows are just toasted.
Remove and allow to cool on the tray for five minutes before transferring to a wire rack to cool completely.