Sift the flour, cocoa powder, icing sugar into a bowl. Rub in the butter until it looks like fine breadcrumbs.
215 g plain flour (all-purpose flour), 35 g cocoa powder, 2 tbsp icing sugar, 125 g butter
Beat the egg yolk with the cold water then pour it into a well in the middle of the flour mixture.
1 egg yolk, 4 tbsp cold water
Use a spoon to mix it together and then press together with your hands.
Cover and put it in the fridge for half an hour.
Remove from the fridge and roll out carefully so it is larger than your tart tin. Drape over the tin and push the corners gently into place. Place your tin on a baking sheet to make it easier to handle.
Then put it back into the fridge to chill for another half an hour. Meanwhile preheat the oven to 180℃ / 350℉.
Fill with baking beans (I used dried pasta) on top of baking paper and bake for 10 minutes. Then remove the baking beans and baking paper and bake for a further 10 minutes for the base to become crisp.
Leave the case to cool. Trim the edges off with a sharp knife.