1. Make pudding: Combine sugar, flour and salt in a medium pan. Slowly whisk in milk. Cook over medium heat, stirring constantly, until mixture just begins to boil, about 8 minutes. Gently boil for 3 minutes.
2. Place yolks in a medium bowl. Whisk to blend. Whisking constantly, slowly drizzle in 1/2 cup of hot milk mixture. Immediately transfer yolk mixture to milk mixture in pan. Cook, stirring constantly, about 2 minutes. Remove from heat; whisk in butter and vanilla. Pour through a fine-mesh sieve into a clean bowl. Cover with plastic wrap, pressing wrap directly onto surface of pudding. Let cool, then refrigerate until cold.
3. Make topping: When ready to serve, whip cream with sugar and vanilla until soft peaks form. Divide 1/2 of pudding among 4 bowls. Top with 1/2 of bananas and wafers. Add remaining pudding, bananas and wafers. Top with whipped cream.