Preheat oven to 175°C fan or 190°C conventional.

Lightly grease a 20 cm baking dish with butter.

Whisk melted butter, brown sugar, and granulated sugar until smooth.

Add egg and vanilla, mix until combined.

In another bowl, mix flour, baking soda, and salt.

Add dry ingredients to wet, mix until just combined.

Fold in chocolate chips and chopped chocolate.

Press dough evenly into dish. Top with extra chocolate if desired.

Bake 18–23 min, until edges are golden and center is slightly soft.

Cool in dish 10–15 min before serving.