Pre-heat your oven (fan) to 180°C/ 356°F. Place baking paper into a large baking tray.
In a standing mixer, combine your butter and sugars (both types) on a medium-high speed for around 4-5 minutes. You can leave this for longer if you wish. Don't cut this step out, be patient and let the sugars cream effectively with the butter. After this step, add the vanilla extract and combine until everything is incorporated. Then, add the remaining ingredients (except the chocolate/ marshmallows). Combine on a medium speed for around 30 seconds, until all combined. Do not over mix at this stage. Finally, add the choc chips and combine on a low speed until they are integrated.
Using a tablespoon, spoon out a cookie dough ball onto your lined baking tray. I like the cookies quite small, but if you want the cookies larger, spoon out more than a tbsp. Ensure that when you place the cookies on the baking tray, they are separated enough. I did around 4 cookie dough balls per baking tray. If needed, you may want to do several batches (I did this!). Press some marshmallows into the top of the cookie. I cut a marshmallow into 4 and used 1 quarter for each cookie dough ball. Bake for 10 minutes.
When you take the cookies out of the oven they may seem slightly under-cooked but they will firm up and be deliciously gooey inside as they cool down! As soon as you take them out of the oven, use a dessert ring or small bowl to circle around the edges of the cookie, ensuring they are circular in shape.
Enjoy the cookies warm! Or store in an airtight container. I hope you love them!