In a large heavy saucepan, mix sugar and cornstarch. Whisk in milk. Cook and stir over medium heat until thickened and bubbly. Reduce heat to low; cook and stir 2 minutes longer. Remove from heat.
In a bowl, whisk a small amount of hot mixture into egg yolks; return all to pan, whisking constantly. Bring to a gentle boil; cook and stir 2 minutes. Remove from heat. Stir in butter, vanilla and salt. Cool 15 minutes, stirring occasionally.
Reserve 1 cup whole wafers and 1 banana for topping. Crush 2 cups wafers and set aside. In a 13×9-in. baking dish, place a single layer of whole wafers, filling gaps with crushed wafers. Layer with a third of the bananas and pudding. Repeat layers twice. Press waxed paper onto surface of pudding. Refrigerate, covered, overnight.
In a bowl, beat heavy cream until it begins to thicken. Add sugar; beat until soft peaks form (do not overmix). Just before serving, remove paper and spread whipped cream over pudding; top with reserved banana, sliced, and wafers.