Adjust an oven rack to the middle of the oven. Preheat the oven to 400 F (200 C). Line two large sheet pans with parchment paper and set aside.
In the bowl of a stand mixer fitted with a paddle, beat the butter on medium speed until creamy, about 1 minute. Add the granulated and brown sugars and the salt, and beat on medium speed until light and fluffy, 2 to 3 minutes, scraping down the sides as needed.
Add the egg, egg yolk, and vanilla, and mix until smooth. I usually add a tablespoon of the flour at this point as well, to keep it from curdling.
Then add the flour, cocoa powder, baking powder, and baking soda and beat on low speed until just combined. I don't usually add all the flour at once, but about two-thirds, mix everything in, then the rest.
Add the chopped chocolates and mix into the batter on low speed until JUST incorporated. Use a rubber spatula (or your hands) to give the dough a final mix and make sure it is completely combined.
Form the dough into small balls, about 2 tablespoons each [50 g], but then flatten them slightly into thick disks. Place them on the sheet pans spread a bit apart. Sprinkle each dough ball with a little flaky salt.
Bake (one pan at a time or both, your call), rotating halfway through baking. You want to bake the cookies until the tops are slightly firm and the cookies are slightly puffed and starting to crinkle...I've found between 8 and 11 minutes is the sweet spot.
Remove from the oven and allow to cool on the pan for a few minutes, then transfer them to a wire rack to finish cooling. These are best slightly warm, but still amazing for at least a few days.