Beat together butter and sugar until light and well combined.
Add sifted flour and mix until it crumbs together.
Add yolks and knead together - add water if needed to bring together into a dough.
Form into a ball, cover in plastic and chill for at least 1 hour.
Roll out pastry carefully and line a 24cm flan tin.
Return to the fridge to chill then blind bake the tart shell until golden brown at 180°c.
Melt chocolate and butter together gently until combined and cool slightly.
In a mixer with whisk attached, whisk the room temperature eggs, yolks and sugar until very, very thick and light in colour.
Turn down mixer speed to medium and gradually & slowly add in the chocolate mixture.
Mix for 20-30 seconds only in the machine, then very gently using a spatula, fold the mix by hand until combined, being careful not to knock out too much of the air.
Pour mixture into blind baked pastry shell and bake for 15 minutes at 120°c fan forced oven.
Allow to cool.