Cream the butter and sugar together until pale
Add the egg yolks and beat into the mix one at a time
Mix in the vanilla, lemon zest and chocolate chunks
In a separate bowl mix together the dry ingredients, then combine with the butter mix until you have a nice soft dough.
Take about a dessert spoonful of the dough and roll in your hands to form a ball.
Flatten slightly, then place on a baking sheet lined with baking parchment
Repeat with the rest of the dough, leaving about 3cm between each cookie.
Bake in a preheated oven at 180deg C (160 for fan ovens) for around 12 minutes, or until the edges have just begun to crisp, but not brown across the cookies.
Remove from the oven and leave to cool.