Whisk the melted butter, granulated sugar, and brown sugar until smooth, about 1 minute. Add eggs and vanilla, whisk until the mixture lightens and thickens, another 1 minute.

Sift in the flour, cocoa, baking powder, and fine salt. Stir with a spatula just until no dry flour remains, about 30 seconds.

Fold in the chocolate chips until evenly distributed.

Scoop into 1.5-tablespoon balls, place on a parchment-lined plate, cover, and refrigerate at least 30 minutes.

Preheat oven to 350°F. Place chilled dough balls on a parchment-lined baking sheet 2 inches apart. Bake 12 to 14 minutes, until tops are crackly and edges set. Sprinkle with flaky sea salt immediately, cool on sheet 5 minutes, then transfer to a rack.