In a 3-quart saucepan (not yet set over heat), stir together the sugar, cornstarch and salt. Gradually add the cold milk, stirring until smooth.
In a second bowl or in a large Pyrex measuring cup, whisk the egg yolks and keep the bowl next to the stove.
Set the saucepan with the milk mixture over medium heat and cook until mixture simmers, stirring often. When mixture starts to simmer, spoon out about ¼ cup of the hot milk mixture and stir it into the egg yolks until smooth. Add the milk/egg yolk mixture back to the saucepan and continue stirring until mixture thickens and boils. Continue stirring and boiling (gently) for 1 minute.
Remove thickened mixture from heat and add butter. Stir until butter is melted, then stir in vanilla. Let cool for about 10-15 minutes before assembling.
Prepare custard cups. Lay a vanilla wafer on the bottom of each custard cup, then add two or slices of banana. Spoon a little less than 2 T. of the vanilla mixture over the wafers and banana, then repeat, but this time layer 2 vanilla wafers, more banana and about 2 t. of the vanilla pudding. Cover cups with plastic wrap and keep chilled until you are ready to make the cream. Note: Chilling will also soften the cookies a bit.
To make the whipped cream, whip cream and sugar until stiff peaks form, then beat in the vanilla. Spoon on top of chilled banana cream pie cups.