In a medium saucepan, whisk together the whole milk, granulated sugar, flour, and salt over medium heat.

Heat the mixture, stirring constantly until it thickens and bubbles gently, about 10 minutes.

In a separate bowl, whisk the egg yolks. Gradually add a little of the hot milk mixture to the yolks to temper them, then return it all to the saucepan, stirring to combine.

Remove the saucepan from heat and stir in the vanilla extract.

In a 9x13 inch dish, layer half of the Nilla wafers on the bottom, then half of the banana slices. Pour half of the custard over the layers.

Repeat with the remaining wafers, bananas, and custard.

Cover the dish with plastic wrap and refrigerate for at least 4 hours or overnight.

When ready to serve, top with whipped cream if desired.