In a medium saucepan, whisk together granulated sugar, flour, and salt. Gradually whisk in the milk until smooth.

Cook over medium heat, stirring constantly, until the mixture thickens and comes to a gentle boil (about 8-10 minutes). Reduce heat to low.

In a separate small bowl, lightly beat the egg yolks. Slowly temper the egg yolks by gradually whisking about 1/2 cup of the hot milk mixture into the yolks.

Pour the tempered egg yolk mixture back into the saucepan with the remaining milk mixture. Increase heat to medium-low and cook, stirring constantly, for another 2-3 minutes, until the custard thickens significantly. Do not boil.

Remove from heat. Stir in the cold butter until melted and fully incorporated. Stir in the vanilla extract.

Pour the custard into a clean bowl. Press a piece of plastic wrap directly onto the surface of the custard to prevent a skin from forming. Chill in the refrigerator for at least 2 hours, or until completely cold and firm.

In a large mixing bowl, using an electric mixer, beat the cold heavy cream, powdered sugar, and 1/2 tsp vanilla extract on medium-high speed until stiff peaks form. Be careful not to overbeat.

In a 9x13 inch baking dish (or individual serving dishes), spread a thin layer of the chilled custard on the bottom.

Arrange a single layer of vanilla wafers over the custard.

Layer about half of the sliced bananas over the wafers, followed by about half of the sliced strawberries.

Gently spread about half of the remaining custard over the fruit layer.

Repeat with another layer of vanilla wafers, the remaining bananas, and the remaining strawberries.

Spread the rest of the custard over the fruit.

Top evenly with the prepared whipped cream.

Cover the dish loosely with plastic wrap and chill in the refrigerator for at least 1-2 hours (preferably 4 hours or overnight) to allow the wafers to soften and the flavors to meld.

Serve cold, garnished with extra fresh strawberry slices or a whole vanilla wafer, if desired.