Whisk together the sugar, cornstarch and salt in a large saucepan; whisk in the milk until smooth. Heat over medium-high heat, whisking, until bubbles begin to form. Remove from the heat.
Whisk the egg in a medium bowl. Slowly pour 1/4 cup of the hot milk mixture into the egg while whisking vigorously; whisk until smooth. Pour the mixture back into the saucepan and cook over medium-high heat, whisking vigorously, until thick and starting to bubble, 1 to 2 minutes. Transfer the mixture to a bowl, whisk in the sour cream and vanilla and let cool until thick, whisking frequently.
Cut the bananas into thin slices. Layer a few banana slices, a few wafer pieces and 2 tablespoons of the prepared pudding in each of 4 glasses; repeat the layering once and then top with a final layer of pudding. Serve immediately or cover and refrigerate for up to 4 hours.