Preheat the oven to 180C (160CFan) and line a baking tray with baking parchment and leave to one side
In a bowl cream together the butter and both sugars until pale in colour and fluffy.
Add in the egg and egg yolk, flour, cocoa powder, vanilla, baking powder, bicarbonate of soda and cornflour and mix until completely combined and a dough begins to form.
Add in the marshmallows and chocolate chips and mix again until they are evenly distributed.
Weight your cookie dough into balls of around 80g-90g and place them on your baking tray or in your freezer-safe container.
At this point, if you want to freeze the cookies pop them into the freezer. Bake within three months. From frozen the cookies should bake for 15-17 minutes.
If you are baking the cookies now, spread them out evenly on your baking tray and bake in the oven for 13-15 minutes or until they start to go golden brown around the edges.
Remove from the oven and leave to cool fully on the baking tray.