If your bananas are not ripe, place them onto a baking tray and into the oven at 150C/300F for 20-30 minutes or until they are blackened. Let them cool slightly, remove the skin and blend the flesh into a smooth paste.
In a pot, bring your milk to a slight simmer. While your milk is heating up, in a separate bowl, add in the egg yolks, sugar and vanilla and whisk vigorously until pale and puffy. Whisk in the cornstarch and then slowly drizzle in the warm milk while whisking.
Transfer the mixture back into the pot and continue stirring over low-medium heat. It should slowly turn into a thick custard. Remove from heat and slowly add in the butter and whisk well until incorporated. Whisk in the banana paste from step 1. Place a piece of plastic wrap over the surface of the custard and refrigerate.
Once the custard is cooled, whip your heavy cream into stiff peak. Fold the banana custard into the cream, transfer the mixture into a piping bag and return to the fridge.