Cream the butter and sugar together until soft and pale.
Add the flour, chocolate and nuts. The dough should come together but add up to a tablespoon of milk if you need to.
Transfer the mixture to a large piece of clingfilm in a flatish shape.
Put the dough in the fridge for 30 minutes to rest.
Line 2 baking sheets with parchment paper or reusable silicon.
Unwrap the dough but leave on the cling film and cover with another piece.
Roll the dough out to about 1 cm thick.
Cut 12 pieces with a knife, about 10 cm square.
Place into an oven preheated to 180 C / 165 C Fan / 350 F/Gas Mark 4 for 10-12 minutes, or until lightly browned.
Let the chocolate shortbread rest on the baking tray for a few minutes then transfer to a cooling rack.
Notes
If your hazelnuts still have the brown skins on pour over boiling water, leave to cool for a few minutes and the skins will slip off. Dry them on paper towel before chopping.