Caramelise brown sugar and butter slightly in a heavy-based saucepan until mixture starts browning.
Remove from heat and gradually add 200 ml heated milk.
Stir to dissolve sugar.
Mix custard powder, salt and remaining milk to a smooth paste.
Add to milk mixture and bring to the boil until thick. Cook for about 3 minutes.
Remove from heat and add essence and liqueur (if using).