Whisk the cornstarch, sugar, egg yolks and salt in a medium bowl heat the milk, 1/2 cup heavy cream, and mashed banana in a medium saucepan over medium heat until bubbles form around the edges. While whisking the egg yolks, add the hot milk a little at a time. When the bowl feels warm, whisk in the remaining milk. Make sure you whisk well.
Return to the saucepan and set over medium heat. Whisk until the custard thickens and then boil for 2 minutes. Put the vanilla and butter in a large bowl and strain the hot cream over them through a fine-mesh sieve. Stir until smooth, then press a piece of plastic wrap directly on the surface and refrigerate until cold.
For the topping: Whisk the 3 cups of heavy cream, powdered sugar, and 2 teaspoons vanilla extract in a large bowl until soft peaks form. Remove chilled pudding from the fridge and stir/fold in half of the whipped cream topping into the pudding with a spatula until completely combined.
Assembly: Slice banana into 1/4 inch thick slices and brush lightly with lemon juice to prevent browning. Spread a thin layer of pudding over the bottom of a 9x13 inch baking dish and top with an even layers of vanilla wafers, banana, and pudding (not the whipped topping though) and repeat as you wish until all ingredients are gone. Top the finished layers with remaining whipped cream, cover and refrigerate for 4 hours.