In a large bowl sift together the flour, cocoa powder, baking soda and salt. Set aside.
Using a mixer fitted with the paddle attachment, beat butter and sugars for 3 minutes on medium speed until fluffy. Scrape down the sides and bottom of the bowl as needed. Beat in the egg and vanilla extract. Add the dry ingredients and beat on medium-low speed just until combined. Fold in chocolate chips. The batter will be thick and sticky.
Optional: Chill the dough in the refrigerator for at least 1 hour or overnight. This will help the cookies stay thick and spread less while baking.
Preheat oven to 350F/180C degrees. Line two large baking sheets with parchment paper. Set aside.
Take cookies out of the fridge. If the cookie dough is hard to handle, leave it at room temperature for 15 minutes.
Drop rounded balls of dough, the size of 2 tablespoons, onto prepared baking sheet, spacing them 3 inches (8 cm) apart. Bake for 9-10 minutes, just until the edges start to set. Leave to cool on the pan for 10 minutes and then carefully transfer to a wire rack to cool completely.
Store cookies in an airtight container for up to 4 days. They can also be frozen for up to 2 months.