Sift the flour, cocoa, salt, baking powder and baking soda in a bowl. Set aside.
In a mixing bowl, beat the butter and sugars with a hand mixer until lightened in colour. Add the egg and vanilla and beat again to combine.
Fold in the flour mixture slowly, then stir in the chocolate chips. The dough is soft and sticky.
Cover the bowl with clingfilm and chill the dough for at least 1 hour. You can chill it for up to 24 hours but I personally don't find that necessary.
When ready to bake, preheat the oven to 175 C and line a baking sheet with a silicone mat.
Using a cookie scoop (highly recommended here) or a spoon, scoop out equal portions of the dough, about 2 tablespoons of dough each. Place them on the mat at least 2 inches apart. You will most likely need to bake in batches so keep the remaining dough back in the fridge.
Bake for 15 minutes until the cookies are set around the edges but the tops still feel soft. Lightly press down on them with a spatula if they look puffy. Press a few more chocolate chips into the tops while the cookies are warm.
Let the cookies cool for 10 minutes, then transfer to a wire rack to cool completely. They will firm up as they cool. Repeat with the remaining dough.
Store the cookies in an airtight tin at room temperature for 4 to 5 days, but in humid areas, I'd recommend putting them in the fridge after day two and they'll last about two weeks that way. Happy baking!