1In a food processor blitz the biscuits to a crumb and add the rest of the ingredients and blitz again until they are all fully incorporated.
2Line a 9" baking tray with greaseproof paper and firmly press the crumb mixture down to the bottom, bake at 180C for 10mins.
3For the caramel: Mix together all the ingredients (apart from the salt) in a non-stick saucepan on a low heat. Let the caramel cook to a deep brown toffee colour, be careful not to let it stick to the bottom or burn. (This may take a while so be patient).
4Add as much salt as you want at the last moment and pour over the biscuit base and pop in the fridge to set.
5Now for the marshmallow layer, bring a saucepan of water to the boil and place a heat-proof glass bowl over the top to create a bain-marie. Put the marshmallows in the bowl and keep stirring till the marshmallows begin to melt and become sticky.
6Add the sugar syrup and stir till the marshmallow fully melts to a smooth consistency. Spoon on top of the caramel and smooth out using the back of a spoon or spatula, don't worry about trying to get it too smooth as it settles out pretty evenly as it cools. Pop back in the fridge to set again!
7Once the marshmallow is set, pour the melted milk chocolate over the top, giving the tin a little shake, from side to side, to even out.
8Sprinkle over some marshmallows and crushed biscuit - using a blow torch toast the edges of the marshmallows & place in the fridge, overnight will give you best results but an hour or two is fine.
9Cut the millionaires shortbread into pieces, we chose to do 1.5 inch squares so they were bite size (ish)!