Beat together sugars and butter using an electric mixer, until consistent clumps form
Add egg to mixture and beat until mixture is consistent and fluffy
Add flour, baking soda and cornflour to the bowl and beat until combined, Do not over mix, just enough to bind ingredients together Fold caramilk pieces into dough
Cover bowl with cling wrap and place in the fridge to set for at least 30 minutes
Once set, form dough into 8 balls and place on a lined baking tray
Use thumb to dent the top of cookie, place 1 marshamallow in the dent and 1 caramilk square, repeat for all 8 cookies
Bake at 180c/355f for 8-10 mins