Melt the dark chocolate and butter using a double boiler or microwave.

Whisk the egg and add the brown sugar, mixing well.

Whip the egg mixture until it turns pale and slightly frothy.

Add the melted chocolate and butter mixture to the egg mixture. Optionally, add vanilla extract.

Sift the all-purpose flour, baking powder, and salt into the mixture and fold gently.

Cover the dough with plastic wrap and refrigerate for 30 minutes to 1 hour.

Scoop the dough using a spoon or cookie scoop onto a baking tray.