Making the Vanilla Pudding : In a saucepan, heat milk, heavy cream, and half the sugar over medium heat until warm but not boiling. In a bowl, whisk egg yolks with the remaining sugar, salt, and cornstarch until smooth. Slowly pour the warm milk mixture into the egg mixture while whisking to temper the eggs. Return the mixture to the saucepan and cook over medium heat, stirring constantly until thickened. Remove from heat, add butter and vanilla extract, and whisk until smooth. Cool pudding slightly before assembling.

Making the Whipped Cream : Add heavy cream, powdered sugar, and vanilla to a mixing bowl. Beat with a mixer until soft peaks form. Chill until ready to use.

Assembling the Banana Pudding : In a trifle dish, baking dish, or individual cups, start with a layer of Nilla wafers. Add a layer of sliced bananas on top of the wafers. Pour a layer of warm vanilla pudding over the bananas, spreading evenly. Repeat layers until ingredients are used, ending with pudding on top. Spread whipped cream over the final layer of pudding. Chill for 2 to 4 hours to allow flavors to meld and wafers to soften.