Whisk sugar, cornstarch, and salt in a medium saucepan. Gradually whisk in milk and egg yolks until smooth.

Cook over medium heat, whisking constantly, until the mixture thickens and bubbles, about 8 to 10 minutes. Cook 1 minute longer.

Remove from heat. Stir in butter and 2 teaspoons vanilla. Press plastic wrap directly onto the pudding surface and cool for 20 minutes.

Beat heavy cream, powdered sugar, and 1 teaspoon vanilla until medium peaks form.

Arrange a layer of vanilla wafers in a 9-by-13-inch dish or trifle bowl. Add a layer of banana slices and one-third of the pudding. Repeat twice.

Spread whipped cream over the top. Cover and refrigerate for at least 4 hours before serving.

Garnish with additional vanilla wafers just before serving, if desired.