Melt the butter and chopped white chocolate in a double boiler. Whisk until smooth and set aside to cool slightly.
In a small bowl, whisk custard powder (or corn starch) in 1½tbsp milk.
Combine remaining milk and sugar in a saucepan. Bring the mixture to a boil. Remove the pan from heat and add the melted chocolate and slowly stir in the custard (corn starch) mixture. Return the pan to the heat and cook on medium flame, stirring continuously, until the mixture thickens. Turn off the heat and add the vanilla extract. Mix well and pour the custard into a bowl.
Place a plastic wrap on top of the custard and refrigerate until ready to use. This can be made at least 2~3 days ahead.