To a large bowl (that's heat safe), set over a pan of simmering water, add the butter, chocolate and cocoa powder. Make sure the water doesn’t boil or touch the bottom of the bowl. Stir frequently until the chocolate is melted and set aside.
While the chocolate + butter are melting: In the bowl of an electric stand mixer fitted with the paddle attachment (*you can also use a bowl and whisk for this recipe), add the brown sugar, sugar, eggs and vanilla and whisk on medium-high for 5 to 7 minutes until light in color, doubled in volume and thick in consistency.
With the mixer on low, stream in the melted chocolate mixing until combined. Take the bowl out of the mixer and stir in the baking powder and salt.
Mix in the flour until just combined. Scrape the bottom and sides of the bowl to make sure everything is thoroughly mixed. Finally add in the chocolate chips, stirring until evenly distributed throughout the cookie dough. Set aside.
While the dough takes a short rest on the counter, preheat the oven to 350°F (177°C) and line several baking sheets with parchment paper.
Use a 3-tablespoons cookie scoop and drop the dough on the prepared cookie sheet leaving 2 inches (5 cm) between cookie dough balls. This dough is sticky. If you don't have a cookie scoop, use two spoons. Add extra chocolate chips (3-5) to the tops of the cookies for them to look like the photos.Immediately place in the center of the oven and bake for 12 to 13 minutes. The cookies will be puffed and then collapse and crack as they cool. Sprinkle with sea salt flakes, if using. Let cool completely on the baking sheets before serving. The cookies will be lighter in color fresh from the oven and will deepen in color over time.