In a bowl, cream melted butter with castor sugar until light and fluffy.
Add in brown sugar and cream again until everything is mixed well.
Add egg in batches of 1 no at a time whisking continuously. Add in vanilla extract and mix.
Now add refined flour and Cadbury Cocoa Powder. Mix well.
Add in roughly chopped Cadbury Dairy Milk into the batter and fold in gently.
Transfer the batter in a greased baking tray and spread evenly.
Arrange Cadbury Dairy Milk pieces on top and bake at 180 degrees for 25 - 30 minutes.
Once cooled, cut into squares and serve.