In a small pot, mix together the custard powder, sugar and vanilla extract. Gradually add cold milk and stir until well blended. Bring to a boil over medium heat. Continue cooking over medium-low heat, stirring constantly until thickens (about 10-11 minutes). Do not leave it unattended. Remove from the heat and let the custard cool down to room temperature.
Arrange a layer of vanilla wafers on the bottom of your dessert dishes (I used 4 dishes).
Cover with a layer of sliced bananas.
Then pour the custard mixture over the bananas.
Refrigerate overnight. Garnish with whipped cream or sliced bananas and serve.