Preheat the oven to 350 degrees. Combine the sugar, cornstarch, milk and salt in a large saucepan. Cook, stirring, over medium heat until thickened. Beat the eggs lightly with a wire whisk. Add a little of the hot mixture to the eggs. Blend well. Pour the eggs into the mixture in the saucepan. Cook, stirring, for a few minutes. Add the butter and the vanilla. Blend well. Cover the surface of the pudding with plastic wrap. Cool slightly. Place a layer of vanilla wafers on the bottom of a (2 Qt.) casserole. Alternate additional layers of vanilla wafers with layers of banana slices and pudding, ending with pudding as the top layer. Bake until set (about 10 minutes).