Preheat your oven to 350°F (175°C).
In a heatproof bowl, combine the chopped bittersweet chocolate and unsalted butter over a pot of simmering water. Melt until smooth, then let cool slightly.
In another bowl, whisk together the flour, cocoa powder, salt, and baking powder. Set aside.
In a large mixing bowl, beat the eggs and sugar until pale and thickened.
Stir in the vanilla extract and melted chocolate mixture into the egg and sugar mixture.
Fold the dry ingredients into the chocolate mixture until just combined.
Gently fold in the semi-sweet chocolate chips.
Drop rounded tablespoons of dough onto a parchment-lined baking sheet, leaving space between each.
Sprinkle a pinch of sea salt on top of each cookie, if desired.
Bake for about 10-12 minutes. The edges should look set, while the centers will still appear soft.
Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.
