In a saucepan over medium heat, whisk together milk, cream, sugar and cornstarch. Cook, stirring constantly, until the mixture is thickened and just beginning to bubble. Add a pinch of salt.

In a separate bowl, whisk the egg yolks. Slowly pour a small amount of the hot mixture into the yolks to temper them, then return everything to the saucepan. Continue cooking until thick and smooth.

Remove from heat, and stir in butter and vanilla. Let cool slightly.

In a chilled bowl, whip the heavy cream with powdered sugar until soft peaks form. Set aside.

In small glasses, start with a layer of crushed vanilla wafers. Add a layer of banana slices, then spoon over the warm custard. Repeat the layers, finishing with custard on top.

Top with a few banana slices, whole wafers and wafer crumbs. Refrigerate for at least 2-3 hours (overnight is even better) so the flavors meld and the wafers soften. Serve chilled.