Line a couple of baking sheets with baking paper and put to one side for now.
In a large bowl, or stand mixer cream together the butter and the sugar until smooth.
Crack the egg in a separate jug and add into the mix, along with the vanilla extract. Mix until well combined.
Add the flour, baking powder and hot chocolate powder and mix until combined.
Using an ice cream scoop or melon baller (or even just spoons if you don’t have these to hand), scoop the cookie dough onto a baking sheet about 2 inches apart. They will spread when baking.
Chill in the fridge for an hour (the cookie dough, not you. You can chill with a cup of tea)
After 45 minutes, preheat the oven to 350 ºF or 175 ºC. Remove the cookies from the fridge and gently press them down a bit.
Bake for 8 minutes and then add the marshmallows. Bake for a further 4 minutes, until the marshmallows start to toast.
Leave to cool for 5 minutes then transfer to a wire rack. Allow to cool completely before removing from the baking sheets.