Preheat the oven to 160deg fan assisted. Have ready a baking sheet lined with a silicone baking sheet or baking parchment.
Cream the butter and sugar together until well combined. This will only take a couple of mins if the butter is soft enough.
In a separate bowl, sieve the flour, cornflour and cocoa powder together. Add the salt and chocolate chips. Give it another good mix.
Add this mixture to the butter sugar mixture. Mix everything together to form a firm dough.
Roll the dough out into a square, onto your prepared tray. Cut into equal portions and place the tray into the fridge for 20 mins. This helps stop the dough from spreading to much during baking, especially if the butter you use was very soft.
Bake in the centre of the oven for 15 - 20 mins. Or until baked. They may still feel slightly soft as you take them out of the oven. But they will firm up once cooled. If you find that a few have spread and stuck together a little during baking. Or some are a little too big for little ones. Wait until they have cooled a little before using a knife to slice through them again. Or cut into smaller pieces. Leave to cool completely.
Using your hands to bring the dough together, may result in the butter becoming too soft because of the warmth of your hands. Best use a wooden spoon or spatula.Having cornflour in the recipe gives these biscuits a nice light, melt in the mouth texture.