Preheat your oven to 350°F (175°C). This ensures that your cookies bake evenly.
Line a baking sheet with parchment paper. This will prevent the cookies from sticking and make cleanup easier.
In a heatproof bowl, combine the coarsely chopped bittersweet chocolate and the pieces of unsalted butter. Melt them together over a double boiler or in the microwave, stirring until smooth and glossy. Set aside to cool slightly.
In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, salt, and baking powder. This step ensures that the dry ingredients are well combined and helps avoid clumps.
In a large mixing bowl, whisk the eggs, granulated sugar, and pure vanilla extract together until well combined. Gradually add the melted chocolate mixture, stirring until everything is incorporated.
Fold the dry ingredients into the wet mixture using a rubber spatula. Be careful not to overmix; you want to combine them until just incorporated.
Gently fold in the semi-sweet chocolate chips, ensuring they are evenly distributed throughout the dough.
Using a cookie scoop or tablespoon, drop dollops of dough onto the prepared baking sheet, leaving enough space between each cookie for spreading.
If desired, sprinkle a pinch of flaky sea salt on top of each cookie for a delightful contrast to the sweetness.
Bake in the preheated oven for 10-12 minutes, or until the edges are set but the centers are still soft. They will continue to firm up as they cool.
Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a cooling rack to cool completely. Enjoy them warm, or store them for later!