Preheat oven to 350°F. Line 2 baking sheets with parchment paper.
Using an electric mixer (with the paddle attachment if using a standing mixer), beat butter and sugar in large bowl for 5 minutes on medium speed. Pause twice to scrape down sides of bowl and beater. Add eggs and beat on medium high until light and fluffy, about 1 minute more, pausing twice more to scrape down bowl and beater.
In a separate bowl, whisk flour, cocoa powder, baking soda, baking powder and salt. Add to mixer on low and beat until just combined. Stir in 3/4 of the chocolate, reserving the rest for topping the cookies.
Form dough into 1-inch balls. For the nicest appearance, roll the dough balls between your palms to make them smooth. Place 2 inches apart on prepared baking sheets.
Bake cookies 6 minutes, then remove them from the oven, bang the pan firmly on the counter to deflate them, and press a few more chocolate chips or chunks into the centers. Continue baking until edges begin to firm but center still appears soft, about 3 more minutes. Cool on baking sheets 5 minutes then transfer cookies to cooling rack.Sprinkle with flaky salt, if you like, while the cookies are still warm.
I find chilling the dough unnecessary for this recipe, but if your first batch spreads, you can chill the rest. (For future reference, that usually means you didn't cream your butter and sugar for long enough or your butter was too soft!)
As soon as they come out of the oven, use an overturned glass to re-shape the cookies into perfect circles (see video).