Preheat oven to 180 degrees celsius (160 fan-forced).
Grease and line an 18X28cm rectangular slice tin with baking paper.
Place butter and dark chocolate melts into a microwavable-safe bowl.
Melt on 50% power for 3-4 minutes, stirring every 30 seconds with a dry metal spoon.
Sift plain flour, salt, baking powder and cocoa into a large bowl.
Add the brown sugar, white sugar and milk chocolate chips and mix together with a wooden spoon.
Whisk the eggs lightly and add to the bowl.
Pour in the melted chocolate/butter mixture.
Mix with a wooden spoon until just combined.
Pour into the prepared tin and bake for 25-30 minutes or until a skewer inserted into the middle comes out clean.