Use a hand mixer to cream butter, brown sugar and caster sugar together in a large mixing bowl. 

Add half the flour, and continue to mix until just combined.  Add the rest of the flour and again mix until just combined.

Use a spatula to mix the chocolate chunks through the dough.

Divide the dough in half, placing each half on a large piece of plastic wrap.  Fold the plastic wrap over the dough to prevent your hands getting all sticky, then shape the doughs into logs.  Rolling the logs on the counter will help it take shape but don't worry about it being perfect.  If you want large shortbread cookies, make your logs around 5 - 7 cms (2-3 in) in diameter.  For mini cookies, make the diameter half that. 

Pop the logs in the fridge for 2 hours, or until completely firm. 

Preheat the oven to 160C / 320F and prepare two baking trays lined with baking paper or silicone mats.

Remove the logs from the fridge and slice into 1.5cm (.5 in) rounds.  Place the rounds onto the baking trays, leaving space in between. 

Bake for 10-15 minutes, until the shortbread starts to turn golden brown, especially around the edges.  Remove from the oven and allow to cool on the baking trays until firm (around 10-15 mins), then move to a wire rack to cool completely.