Preheat oven to 350ºF. Combine butter, sugars, salt and baking soda in the bowl of a stand mixer. Start on low speed until it comes together, then increase to medium-high and beat for a FULL 5 MINUTES, pausing a couple of times to scrape down sides of mixer bowl and beater. (This 5 minute beating is key to getting enough air in the cookie to insulate the fat and prevent too much spreading)Beat in egg and vanilla extract. Once the egg is in, you do not want to beat it much, just to mix in.
Pause the mixer and add the flour. Turn it on low and beat until flour is fully incorporated, scraping down mixer bowl again once or twice. Don't over-beat or the cookies get harder.Add chocolate chips and stir in with mixer speed on low.
Scoop dough into two-tablespoon-sized balls. Pinch the balls upwards to make them tall (like snowmen). This, again, prevents too much spread.Place dough balls on parchment-lined baking sheets 3" apart and bake 8-11 minutes. Edges should be golden while centre still appears soft and slightly paler.
Immediately after taking the cookies out of the oven, bang the pan on the countertop to deflate them. You can use an overturned glass or small bowl to gently reshape the cookies back into a perfect circle.Top cookies with a few extra chocolate chips, gently pressing them in (this helps to flatten the cookie more). Let cool 2 minutes before transferring to wire racks. Sprinkle with a few grains of flaky salt if desired.
Notes
If you have a kitchen scale, tap the metric button in the recipe card. I prefer to weigh it for accuracy and consistency.
Use a fresh baking sheet between batches or run the used one under cold water (if you don't mind that it may warp!) and dry it before making another batch (a still-warm pan will make the cookies spread too much).
See notes in post for freezing instructions + option to bake dough from frozen.