Clean the mint leaves well and rub with the sugar for about 5 minutes.
Mix this mint- sugar into the milk in a saucepan.
Heat while stirring occasionally until milk is warm and all the sugar is melted
Take off heat just before it boils, cover and let rest for 20 minutes.
Strain this milk to remove the leaves .
To the strained milk , sift in the cocoa , corn flour and salt.
Heat on medium , while stirring often until the mixture comes to a boil
Lower heat and simmer for 3-4 minutes while stirring continuously until mixture thickens. Make sure that you scrape the sides and bottom of pan often while heating to avoid lumps.
Take off heat , stir in the butter and vanilla ( optional)
Strain or pour into individual bowls or a single dessert dish that holds about 2 cups
Serve warm or refrigerate until cold. If you don't like the thick layer that forms on custards , cover the top with cling film (touching the pudding surface ).