1. Melt the butter and blitz the biscuits in a blender. You can crush by hand if needed but ensure they are really well crushed.
2. Mix the biscuits with the butter and dark brown sugar. Press into your tin, ensure you press some up the sides as well (about 3/4 the way up). Place in freezer.
3. Blitz or crush 3/4 of your bag of M&Ms, ensuring some are left whole.
4. Fold together the cream cheese, icing sugar and vanilla. Don't mix too much as it'll loosen the mixture.
5. Whisk the double cream to medium peaks (not loose so it's dripping off the whisk but not really stiff). Be careful not to overwhisk.
6. Fold the cream cheese into the double cream. Fold in the M&Ms. Pour onto the frozen biscuit base and level. Refridgerate for at least an hour. Overnight is better.
7. Once refridgerated, melt down the nutella in a pan (low heat) and pour over the cheesecake. Level quickly as it may start to set. Decorate with leftover M&MS. Chill for 30 minutes and serve.