In a medium bowl, cream the butter and sugars, beating with a hand mixer until it's all light and creamy, about 3 to 5 minutes. Add the egg and vanilla extract and beat for an additional minute.
In a small bowl, sift the flour and cocoa powder together to remove any lumps, then whisk in the baking soda and salt.
Add the dry ingredients to the butter mixture and beat at a low speed until the dough is just combined, stopping to scrape the bowl as necessary. Fold in the chocolate chips, holding some back to press in later.
Use a small cookie scoop to portion out one tablespoon of the dough into your hand, then roll into balls, about 1½-inches wide.
Space six balls on each cookie sheet (the cookies will spread a bit). Press a few of the reserved chocolate chips in the top of each cookie.
Bake for 10 minutes until the cookies look set, rotating the pans half-way through.
Let the cookies cool on the baking sheet for 2 minutes, then use a large spatula to transfer them to a cooling rack to cool completely. Serve and enjoy!
After you've portioned out the cookie dough into balls in Step 5, you can freeze them on a rimmed baking pan lined with wax paper for about 30 minutes, then transfer to a ziplock bag for storage. It isn't necessary to defrost frozen cookie dough, just bake at 325°F for 13 to 15 minutes, or until the cookies look set.Baked cookies last 2 to 3 days at room temperature (if they last that long!), and for much longer in the freezer. Frozen cookie dough can last up to 3 months in the freezer.