First off carefully melt the white chocolate with the milk in the microwave until melted and combined. Set aside to cool.
Then, add the butter, brown sugar, cocoa and salt in a microwave safe bowl and microwave these ingredients until melted and combined – stirring as you go.
Add the vanilla essence and eggs, mix through.
Add the flour, fold it in.
Transfer the batter to the baking tin and add spoonsful of melted white chocolate onto the batter, giving it a swirl with a metal spoon into the batter.
Bake for 12 minutes at 180 degrees Celsius – remove from oven and leave to cool before cutting it into blocks.