In a medium saucepan whisk milk, sugar, egg yolks, cornstarch, and salt. Heat on medium heat stirring every couple of minutes until thickened (9-10 minutes). Turn off the heat and stir in the vanilla and butter.
Assembly
In the baking dish, layer vanilla wafers on the bottom (about 25) and then half of the banana slices. Pour half of the pudding mixture over the top then spread it out with a spatula.Repeat the layers with vanilla wafers, banana slices, and pudding, then top with crushed wafers.Alternatively, you can use individual cups. Layer 3-4 vanilla wafers, 4 banana slices, then pour pudding over to cover. Repeat those layers.
Cover and refrigerate for at least 2 hours prior to serving.
Store in an airtight container in the refrigerator for 3-4 days.