Mix together sugar and cornstarch in a medium saucepan. Whisk while pouring in milk slowly into the pan, to prevent clumps.
In a small bowl beat egg yolks. Set aside.
Cook the milk mixture over medium heat, stirring constantly.
Once the milk mixture begins to thicken, add a 1/4 of warm milk to the egg yolks VERY slowly while mixing constantly. (This makes sure you don't scramble your egg yolks.)
Add the tempered eggs into the saucepan, whisking constantly. Cook for 2 minutes.
Remove the saucepan from heat. Add vanilla and butter. Stir until butter is completely melted. Allow it to cool to room temperature.
Assembly
Line the bottom of a mason jar with vanilla wafers and then a layer of banana slices.
Top the bananas with pudding, enough to cover the bananas. Alternate layering the bananas, wafers, and pudding until you've almost filled the jar.
Top the jar off with whipped cream. Cover jar with lid and place in the fridge until cold. Garnish with extra banana slices and wafers, if desired.
Video
Notes
Yield will vary depending on the size of your mason jars. I like the 8-ounce jars for mini desserts.
You can assemble this recipe in an 8-inch x 8-inch baking dish instead of jars if you prefer.