In another bowl, whisk together flour, cocoa powder, baking powder and salt. Set aside.
Using a stand mixer fitted with a whisk attachment or a hand mixer, whisk eggs and brown sugar on high speed for 4-5 minutes. Add chocolate mixture and mix on low speed just until combined. Add flour mixture and mix just until combined. Use a rubber spatula to scrape the sides and bottom of the bowl. The mixture will be soft, so place it in the fridge for 20-30 minutes until it firms up a bit (if the dough stays in the fridge longer than that, it will start to lose its shine, and you're less likely to get shiny crackly cookies, however if it's not an issue for you, you can keep it in the fridge for longer or overnight).
Preheat oven to 350F/180C degrees. Line 2 baking sheets with parchment paper. Set aside.
Using an ice cream scoop drop rounded balls of dough (or about the size of 1.5 tablespoons, you should get approximately 20 cookies) onto a prepared baking sheet, leaving 3 inches between each cookie to allow spreading. Sprinkle each cookie with a little flaked sea salt, if desired.
Bake until cookies are crackly yet soft in centers, 10-12 minutes (I bake the two pans separately). Allow cookies to cool on baking sheets until they are easy to handle (they will be soft, so give it at least 20 minutes), then transfer cookies to a wire rack to cool completely.
Cookies can be stored in an airtight container at room temperature for 2 days, in the fridge for 5 days, or they can be frozen for up to 2 months. Thaw them on the counter, still covered, or overnight in the fridge.