In a large bowl, sift the flour, cocoa, baking soda and salt. Set aside.
Beat the butter and sugars in a mixing bowl using a hand mixer, until light and fluffy. Add the egg and vanilla. Beat to combine.
Gradually add the flour mixture and fold to combine. Finally, fold in the chocolate chips. The dough will be thick and sticky.
Cover with clingfilm and chill for 30 minutes to an hour. I found that a longer chilling time resulted in much thicker cookies and I prefer them to spread a little bit in the oven. If chilling for longer, let the dough soften for about 15 to 20 minutes at room temperature before baking.
Spread the icing sugar out onto a plate. Preheat the oven to 175 C and line a baking tray with a silicone mat. If you're baking in batches, keep two trays ready.
Using a cookie scoop (highly recommended for this dough) or two spoons, scoop equal portions of the dough, about 2 tbsps per cookie. Roll them in the sugar until well coated and rounded, then place on the baking tray, an inch apart. Flatten lightly with the back of a spoon.
Bake for 8 to 10 minutes then remove from the oven and let them cool on the baking tray to firm up. They will be cracked and have crinkled sugary tops. They will look super soft at first, but will cook through on their own. Over-baking will lead to the sugar melting and dissolving.
Let the cookies cool completely, then store at room temperature in an airtight tin for 1 to 2 days, after which they're best in the fridge especially in humid environments. Happy baking!