Pour 2 cups of water in a large saucepan. Add 3 cups of milk, butter, sugar and vanilla. Stir constantly while bringing mixture to a slow boil.
In a separate bowl, mix remaining cup of milk with cornstarch then add egg yolks. Add mixture to contents in saucepan, and cook until it thickens, stirring constantly. Remove from heat and cool.
Layer vanilla wafers in the bottom of a baking dish. Add bananas on top of wafers then add a layer of pudding. Repeat layers of wafers, bananas, and pudding. Bake for 8 minutes then set to cool.
Serve with whipped cream/topping and garnish with banana slices.Makes 6 to 8 Source: