Whisk dry ingredients: In a medium saucepan (off heat), whisk sugar, cornstarch, and salt until combined. This prevents clumps later.
Add dairy and yolks: Slowly whisk in the milk and heavy cream until smooth.
Whisk in the egg yolks. The mixture should be fully combined with no streaks.
Cook the custard: Place the pan over medium heat. Whisk constantly until the mixture thickens and gently bubbles, 6–8 minutes.
Lower the heat if it starts to stick or simmer too fast.
Finish with flavor: Remove from heat. Whisk in butter, vanilla extract, and 1 teaspoon banana extract to start. Taste and add more banana extract a few drops at a time.
You want a warm, natural banana flavor—not candy-like.
Optional flavor boost: For a stronger banana note, whisk in 2–3 tablespoons instant banana pudding mix while the custard is still hot. This is optional but helpful if you like a more pronounced flavor without bananas.
Cool slightly: Transfer the custard to a bowl. Press plastic wrap directly onto the surface to prevent a skin.
Let it cool to lukewarm, about 20–30 minutes.
Make the whipped cream: In a chilled bowl, beat the cold heavy whipping cream and powdered sugar to medium peaks. It should be soft but hold shape.
Fold and lighten: Stir the custard to loosen it. Fold in half the whipped cream to make it lighter and silkier.
Reserve the rest for topping.
Layer the dessert: In a 2–3 quart dish or trifle bowl, add a layer of vanilla wafers. Spoon over a third of the custard. Repeat layers—wafers, custard—until you’ve used it all, ending with custard.
Top and garnish: Spread the remaining whipped cream on top.
Add crushed wafers or a pinch of cinnamon if you like.
Chill to set: Cover and refrigerate for at least 4 hours, preferably overnight. The wafers soften and the flavors meld as it chills.
Serve: Scoop or slice, depending on how long it’s chilled. The texture should be creamy with tender, cake-like wafers.