Sift together cocoa, salt, flour, and icing sugar, then mix the dry ingredients well.
Sifted dry ingredients...
Chop the cold butter, and rub the butter into the dry mixture using a food processor or fingertips. Keep rubbing in until the mixture looks and feels like fine breadcrumbs. This is so that, when the pastry is rolled out, it’s smooth without any butter lumps.
Using a fork, whisk together the egg yolk with the water. Add this mixture to the breadcrumb-like butter mixture to make a dough.
Gather the chocolate dough in a ball and wrap it in plastic wrap. Chill in the refrigerator for about an hour (or in the freezer for 10-20 minutes for a faster chill).
Ready to chill
After the dough has chilled, sprinkle flour over a flat cutting board and use a rolling pin to roll the dough out to about half a cm thick.
Ready to roll
Grease either a full sized tart tin (for a large tart) or cupcake tin (for individual mini tarts). Line the tart tin or cupcake baking tray with chocolate pastry.
Prick the tart shells with a fork. Preheat the oven to 180 degrees C.
Line the tart shells with baking paper and baking beans or rice and blind bake the shells for 15 minutes.
Remove baking tray, allow to cool, and chill.
Juice all the lemons.
Whisk sugar and eggs in a heatproof bowl.
Add the lemon juice and whisk until well combined.
Place the bowl over a saucepan of simmering water, making sure that the water doesn’t touch the base of the bowl.
Heating the lemon curd to make lemon custard
Cook, whisking, for 15 to 20 minutes or until the lemon mix becomes think and custard-like. (note – do not allow the custard to boil at any point).
Remove the lemon mix from the heat and, while still hot, quickly whisk through the butter.
Completed lemon curd after butter has been whisked through
Once the tart cases are chilled, fill each with lemon curd. Cover the completed tarts in plastic wrap and refrigerate overnight. Dust with icing sugar to serve.