Sift the flours and cocoa powder together. Set aside.
Tip the butter into the bowl and whisk with a wooden spoon for a minute to lighten.
Add the sugar and vanilla and whisk for another minute.
Tip in the flour mix and work with the wooden spoon, then your fingers to form a dough.
Lightly mix in the chocolate chips, if using. Don't knead the dough, use a light touch, and just your fingers, not the palm of your hand.
Roll the dough out into logs, wrap in clingfilm and chill for 15 minutes in the fridge.Preheat the oven to 160˚C/310˚F.
Take 1 log out of the fridge, and using a sharp knife, slice little discs about ½ an inch thick (just over 1 cm). Work quickly, as the dough warms up, it'll soften.
Arrange the sliced dough on a baking sheet and bake for 15 minutes.Repeat with the other dough. Or, you can slice the other log, fill another baking tray and bake 2 baking trays at the same time. The tray on the lower shelf may need 2 minutes longer.Cool completely and store in an airtight container.