In a saucepan, heat the milk and water until boiling and remove from the heat
In a bowl, beat together the egg, egg yolk, sweetener, cocoa powder and cornflour until combined. Pour over half of the hot milk very slowly whilst whisking
When mixed, return the mixture to the pan and place on a low heat, whisking constantly until thickened
As soon as the mixture starts to thicken, take off the heat and continue to whisk - the mixture should be thick. Stir in the peppermint extract
Place the chocolate custard into a bowl and cover with cling film so that the film actually touches the top of the custard to prevent a skin from forming. Put into the fridge and chill for 30 minutes
Cut the filo pastry into 12 squares. Spray a muffin tin with low calorie cooking spray and put one square of filo into each of the 6 muffin cups. Spray again with the low calorie cooking spray and put the remaining filo squares on top of the first ones. These don't have to be neat!
Once the chocolate custard has chilled, spoon into the filo cases
Top with the chocolate chips - it works out at approximately 6 chips on each tart. Bake in the oven for 15 minutes or until the filo is golden. Remove very carefully from the tin and serve hot or cold