Preheat the oven to 180°C (350°F) and line a 9"x13" baking pan with baking paper and set aside.
In a medium sized saucepan, melt the butter and condensed milk together. Once melted remove from the heat and stir in the brown sugar and vanilla extract.
Stir through the cocoa, flour and baking powder and then stir through the white chocolate chips. Pour into the prepared baking pan.
Bake for around 30 minutes or until the sides are set and the middle has the slightest wobble.
Serve warm or place in the fridge to become ultra chewy!