Easy quick chocolate custard

Makes 4 small bowls.
Ingredients:
2/3 cup dark sugar
1/3 cup cocoa powder
2 cups milk
2 heaped spoons custard powder
some vanilla essence / coffee decoction / rum (optional)
1 tbsp butter

Mix the sugar and cocoa powder. Slowly add milk over heat. Keep stirring till it comes to a boil. Mix custard powder with a little milk to form a paste. Add this paste to boiling mixture and keep stirring till it thickens. Lower heat, add butter and vanilla essence / coffee / rum. Mix till everything is dissolved.
Pour into containers and chill.