Preheat oven to 180c/350F, 10-15 minutes before they're ready to bake.
Line 2 flat cookie sheets/trays with parchment/baking paper.
Place butter, brown sugar and white sugar into the bowl of the mixer or bowl for hand mixer and beat until light and creamy.
Add in egg and vanilla and beat till combined.
Add in flour, bicarb soda, salt and corn starch/flour and beat until mixed through.
Place chocolate chips and marshmallows into the dough and mix through with a spatula or spoon until the chocolate is dispersed throughout the dough.
Take tablespoons of mixture and roll into balls to make 24 cookies. Alternatively, make 2 tablespoon balls to make 12 cookies (bake for 2 minutes extra)
Place onto prepared cookie sheets leaving about 1 inch between each cookie to allow for spreading. Place some reserved chocolate chips and marshmallow pieces on the top of cookies if using.
Place in the oven for 8-10 minutes.
Cookies are ready when the edges are starting to turn golden in colour and the centres are still soft. The cookies will continue to cook and set on the tray after baking so it's best to take them out before they are baked through.
Leave these to cool slightly, these taste great cold but even better when warm. If they've cooled completely, heat them up in the microwave for a few seconds.