
Preheat oven to 350°F (177°C). Line a baking sheet with parchment paper.
Melt chopped chocolate and butter together in a heatproof bowl set over simmering water (or microwave in short bursts). Stir until smooth, then let cool slightly.
In a separate bowl, whisk the egg and both sugars until thick, pale, and bubbly, about 2–3 minutes. Whisk in vanilla.
Fold the melted chocolate mixture into the egg mixture until smooth.
Sift in cocoa powder, baking powder, and salt. Stir gently until just combined. The dough will be thick.
Fold in chocolate chips or chunks.
Drop heaping tablespoons of dough onto prepared baking sheet, using a lightly greased ice cream scoop, spacing 2 inches apart.
Bake 10–12 minutes, until tops are shiny and cracked but centers are still soft.
Cool on the baking sheet for 5 minutes before transferring to a wire rack.