In a large bowl, rub the butter and flour together until it resembles bread crumbs.
Add the caster sugar and cocoa powder and mix to combine.
In a small bowl, melt the dark chocolate (over a bain marie or in short bursts in the microwave). Whilst stirring the melted chocolate, gradually add the milk to create a ganache.
Add the ganache to the dry ingredients and mix. Add the water a spoonful at a time and mix to create a stiff dough.
Turn onto a floured surface and knead for a couple of minutes before wrapping in cling film and setting aside in the fridge for 30 mins.
Generously grease a 21cm tart tin with butter and pre-heat your oven to 170°c (or 150°c for fan assisted ovens or Gas Mark 3). Place a tray large enough for your tart tin to sit upon into the oven to warm up too.
Roll your dough into a circle large enough to drape into your prepared tin with a small amount of pastry hanging over the edge. Line the pastry with greaseproof paper and fill with baking beans. Bake for 20 minutes on top of the warmed baking tray before removing the baking beans and continuing to bake for a further 10 minutes.