Mash the bananas into a puree. Put the milk in a container on the stove to boil with the vanilla. Separately, beat the egg yolks and sugar until they whiten, add the flour, mashed bananas and whip some more.
Gradually, while stirring continuously, pour the milk into the egg mixture, being careful not to curdle it. Return it to the heat and stir it for 4-5 minutes until the cream thickens. Remove from the heat and stir the cream with the butter.
In dessert bowls, place slices of bananas on the bottom and pour the finished cream in. Put it in the fridge for 2-3 hours before serving.
Optionally, you can beat the egg whites into thick snow and mix them in with the cooled cream, before putting the cream into bowls.