Preheat oven to 175°C (350°F). Grease and line a small square baking tin.

In a bowl, mix melted butter and sugar.

Add eggs and vanilla, whisk until smooth.

Sift in flour, cocoa powder, salt, and baking powder. Mix gently until just combined.

Fold in chopped chocolate or choco chips if using.

Pour batter into the prepared tin and spread evenly.

Bake for 20–25 minutes, or until a toothpick inserted in the centre comes out with a few moist crumbs.

Cool completely, then slice into squares.