Preheat oven to 180C / 350F /160C fan forced. Line 2 large baking trays with baking paper.
Sift together the flour, baking soda and salt.
In a separate bowl, beat together the butter and sugar until lightened and fluffy. Scrape down the sides of the bowl and add half the flour mix. Beat until just combined. Add the milk and remainder of the flour and mix until it just starts to come together. Finally, add the blueberries and choc chips and give the mix another beat until mixed through and the dough is clumping.
Pull the dough together with your hands then take approximately 1.5 to 2 tablespoons full of mixture, roll into a ball and place on a baking sheet 2 inches apart (or you can use a mini ice cream scoop and just scoop it onto the baking trays). Once you have filled a tray lay a sheet of baking paper over the top and use a second baking tray to press down all over to flatten the cookie out to about 5-6mm thick.
Bake for 6 minutes. Sprinkle with a little extra caster sugar, turn the trays and bake for a further 6 minutes. Make sure they are nice and golden around the edges – this gives lots of extra flavour.