Mix cornflour with about a tablespoon of the milk in a small bowl or cup until dissolved.
Heat the rest of the milk in a pot over low heat.
Once milk is simmering, slowly add the cornflour mixture while whisking.
Keep whisking until custard thickens (should thicken fairly quickly).
Remove from heat and add mashed banana and cinnamon. Stir until combined.
Let cool for a few moments and serve.
You could also use berries, vanilla essence or honey as an alternative to the banana and cinnamon.