Preheat the oven to 160°C Fan.
Put the butter into a 23cm oven-proof skillet (ideally cast iron) set over a low-medium heat and stir until melted. Pour in the condensed milk and whisk until well combined.
Remove from the heat and stir in the brown sugar, vanilla extract and a pinch of salt.
Sift in the flour and baking powder, then stir well until just combined - take care not to overmix. Leave to cool for 5 minutes top stop the chocolate chips from melting too much when you add them.
Stir in the chopped chocolate until evenly distributed throughout the dough. Spread in a smooth layer.
Bake for around 20 minutes - or until the sides are set but the middle still has the slightest wobble (remember, it will continue to cook as it stands).
Remove from the oven and leave to stand for 5 minutes. Serve warm, straight from the skillet, topped with scoops of vanilla ice cream. Alternatively, leave to cool completely before covering and chilling in the fridge until firm. You can then cut it into deliciously chewy slices or bite-sized pieces to enjoy as a snack.