Preheat oven to 220°C and spray a 24cm pie tin with cooking spray. Set aside.
Add flour, icing sugar and butter to a food processor and blitz until crumbly. Add egg yolk and water and blitz until the dough forms a ball. Wrap in plastic wrap and refrigerate for 30 mins.
Place the dough between two pieces of baking paper and roll out to a 1cm thick circle. Carefully roll the dough around a rolling pin and gently unroll it over prepared pie tin. Press the dough into the pan and cover with baking paper and fill with baking beans or raw rice.
Bake for 15 mins and carefully remove baking beans and paper. Bake for a further 10 mins and remove from oven. Allow to cool for at least 10 mins. Set aside.
For the Filling
Add sugar, salt and milk to a medium saucepan on medium heat and whisk to combine. Bring to a gentle simmer, whisking frequently.
Meanwhile, add cornflour and egg yolks in a small bowl and whisk until until smooth and well combined.
Once the milk mixture is simmering, add a few spoonfuls of the hot liquid to the egg yolks, whisking after each spoonful to temper the eggs. Add a few more spoonfuls, then slowly whisk the egg mixture into the saucepan. Whisk constantly until mixture begins to thicken and bring to a gentle boil.
Remove from heat and stir through chocolate, butter and vanilla until completely smooth. Pour the filling into the cooled pie crust and refrigerate for 3 hours or overnight.
For the Topping
Add cream and icing sugar to a mix master and beat on high speed until the cream forms soft peaks. Spread whipped cream over pie. Using a microplane or lemon zester, grate chocolate over cream and serve.