If using a boxed pudding mix, prepare the mix according to the directions on the package and refrigerate until ready to assembly.
If making from scratch, whisk together 1/2 cup of the cornstarch with the milk. Make sure it's thoroughly combined and set aside.
In a separate bowl, whisk up the egg yolks, set aside.
In a small saucepan, whisk together the milk, salt, and sugar over medium heat until you see little bubbles around the edges. Add about 1/2 cup of the warmed milk into the whisked egg yolks, and then add it back into the saucepan (we don't want to cook the eggs here, just heat them up to a safe temperature).
Whisk constantly while adding in the slurry (cornstarch and milk mixture) until it starts to thicken a lot.
Add in the mashed bananas, butter, vanilla, and lemon juice. Add to a glass dish, cover with plastic wrap up to the pudding line, and let set up overnight.