In a large bowl combine the egg yolks, sugar, and corn starch. Whisk together until well mixed and pale yellow..

Add the milk to a medium saucepan and heat over medium-high heat until the milk starts to foam up (don’t let it boil).

Remove the milk from the heat and slowly drizzle into the egg mixture, whisking the whole time.

Pour the custard back into the sauce pan and cook over medium heat, whisking constantly, until the mixture begins to bubble. Cook for an additional 60 seconds and then remove from the heat.

Add the butter and vanilla extract and whisk together.

Place a fine mesh sieve over a bowl large enough to hold the custard. Pour the custard through the fine mesh sieve to remove any lumps.

Spoon a small amount into the bottom of an 8 x 8 inch glass casserole dish. Cover the bottom with a single layer of Nilla wafers then top each wafer with a slice of banana. Top with approximately ⅓ of the custard.

Repeat two more times, wafers, banana, custard.

Top with one more layer of wafers.

Cover and refrigerate for at least 2 hours up to overnight.

Serve as is or add a Cool Whip or whipped cream topping before serving.