Drain the pineapple, reserving juice. Puree the bananas with the pineapple juice until smooth. In a small saucepan, bring the puree to a boil, lower heat and cook about 5 minutes. Mix the cornstarch and water to form a smooth paste, then wisk into the puree. Continue heating and stirring until mixture is very thick. Add vegan sugar to taste, stir in pineapples and spoon into custard cups. Serve warm.