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Take out and set aside 1 biscuit
Process the rest of the biscuits and chocolate in a food processor until they start to break down into a crumb mixture, then add the butter and carry on processing until it starts to clump together.
Press into a loose-based, high-sided tart tin and make a smooth base and sides with your hands or the back of a spoon. Put into the fridge to chill and harden.
Melt the marshmallows in a saucepan with the milk over a gentle heat, once the milk starts to foam (not boil) take off the heat and keep stirring until the marshmallows blend into the milk to make a smooth mixture
Pour the mixture out of the saucepan into a heatproof bowl, and then whisk in the crème de menthe and crème de cacao. Leave to cool
Whisk the cream until it starts to hold soft peaks then, still whisking, add the cooled marshmallow mix. This filling should be thick but still soft, not stiff or dry, so that it will eventually drop easily out of the bowl into the chilled pie crust. (NOTE - over whisking at this point can result in a 'separation' type effect and the mixture won't be so smooth - so if in doubt don't whisk too much.)
Once the marshmallow mixture and cream are combined, add a few drops of food colouring (unless you prefer not to) and gently mix inches (remember earlier warning re over whisking!).
Spread the filling into the chilled base tin, swirl it about with a palette knife or a spatula to fill evenly and put back in the fridge, covered, to chill overnight or for a minimum of 4 hours until firm.
Crush the biscuit you put aside earlier and sprinkle over the top of the pie before slicing.
The pie can be made 1–2 days ahead. When chilled and firm, tent with foil (try not to touch surface with foil as it will leave marks) and store in fridge. Decorate just before serving. The pie will keep 3–4 days total. Freeze Note The pie can be frozen for up to 3 months. Open-freeze undecorated pie just until solid, then wrap pie (still in its tart tin) in double layer of clingfilm and layer of foil. To defrost, unwrap pie and tent with foil (try not to touch surface with foil as it will leave marks), then defrost overnight in fridge. Decorate before serving.