Whisk the sugar, cornstarch and salt in a saucepan, then gradually whisk in the milk until smooth.
Cook over medium heat, whisking constantly, until the pudding is thick and bubbling, about 8 minutes, then cook 1 minute more.
Remove from the heat and stir in the butter and vanilla. Press plastic wrap onto the surface and cool for 20 minutes.
In a trifle dish or 9x9 inch dish, layer half the wafers, half the banana slices and half the pudding, then repeat the layers.
Cover and chill for at least 4 hours so the wafers soften into a cake like texture.
Whip the cold heavy cream with the powdered sugar to soft peaks and spread over the top.
Just before serving, garnish with crushed wafers and a few fresh banana slices.
