01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - Set a heatproof bowl over a pot of simmering water. Melt chopped dark chocolate and butter together, stirring until smooth. Remove from heat and let cool slightly.
03 - In a medium bowl, whisk together flour, cocoa powder, baking powder, and sea salt.
04 - In a large bowl, whisk together granulated sugar, brown sugar, eggs, and vanilla extract until pale and thick, about 2-3 minutes.
05 - Pour the melted chocolate mixture into the egg mixture and whisk until fully combined.
06 - Fold in the dry ingredients using a spatula until just incorporated. Do not overmix.
07 - Gently fold in the chocolate chips.
08 - Using a medium cookie scoop or two spoons, drop heaping tablespoons of dough onto the prepared baking sheets, spacing them about 2 inches apart.
09 - Sprinkle with flaky sea salt, if desired.
10 - Bake for 10-12 minutes, until the tops are shiny and cracked but the centers are still soft.
11 - Let the cookies cool on the baking sheets for 10 minutes, then transfer to a wire rack to cool completely.