In a medium saucepan, whisk together the milk, sugar, cornstarch, and salt over medium heat.

Cook the mixture until it thickens, stirring constantly to avoid lumps.

In a small bowl, whisk the egg yolks. Gradually add a spoonful of the hot milk mixture to the yolks while whisking quickly to temper them.

Pour the tempered yolks back into the saucepan and cook for 2-3 more minutes, stirring constantly.

Remove the saucepan from heat and stir in the butter and vanilla extract. Let it cool slightly.

In a separate bowl, whip the heavy cream and powdered sugar until stiff peaks form.

In serving cups, layer the vanilla wafers, banana slices, and pudding. Repeat the layers until the cups are filled.

Top with whipped cream and garnish with additional banana slices or crushed vanilla wafers.

Refrigerate for at least 1 hour before serving to allow the flavors to meld.