Cream together the butter and sugar with a wooden spoon until light and creamy. Add the egg yolks and 35 ml/ 2⅓ tbsp water and mix well until all the liquid has been incorporated.
Gradually add the flour and cocoa powder until a paste is formed. This can all be done in an electric mixer or food processor if you prefer.
Wrap the pastry in foil or clingfilm, flatten it with your hand and refrigerate for 1 hour. The pastry can be made 2 days ahead, or even longer since it freezes well, though in this case you may want to knead the pastry to soften it before rolling.
Sprinkle your surface with flour and roll out the pastry until it is about 5cm (2 inches) bigger than your tart ring or tin. I recommend using a ring that is 24cm (9 or 10-inch) in diameter and 2.5cm (1 inch) deep.
Carefully line the ring or tin, pushing the pastry well into the bottom edges and folding the excess over the top. Trim off the excess.
Refrigerate for 15 minutes to relax the pastry and to help prevent it from shrinking during cooking.
Preheat the oven to 180°C/350°F/gas 4. Line the tart with a sheet of parchment paper – scrunch it up first to soften it – then tip in baking beans, rice or lentils and bake blind for 20 minutes.
Lift out the beans and bake for a further 5 to 8 minutes, until the base is dry. Allow to cool.