**Preheat Oven:** Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
**Melt Chocolate and Butter:** In a heatproof bowl, melt the butter and chocolate together over a double boiler or in the microwave in 30-second intervals, stirring until smooth. Let cool slightly.
**Combine Sugars and Eggs:** In a large bowl, whisk together the granulated sugar, brown sugar, and eggs until pale and creamy. Stir in the vanilla extract.
**Add Chocolate Mixture:** Gradually mix the melted chocolate and butter into the egg mixture until combined.
**Mix Dry Ingredients:** In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture.
**Chill Dough:** Cover the bowl and refrigerate the dough for 30 minutes to 1 hour, or until firm.
**Portion Dough:** Scoop tablespoon-sized portions of dough onto the prepared baking sheet, spacing them about 2 inches apart.
**Bake:** Bake for 10–12 minutes, or until the cookies are set around the edges but still soft in the center.
**Add Sea Salt:** Remove from the oven and immediately sprinkle the tops with flaky sea salt. Let cookies cool on the baking sheet for 5 minutes before transferring to a cooling rack.