Prepare the pudding base: In a medium saucepan, whisk together sugar, cornstarch, and salt. Gradually add milk while whisking constantly to prevent lumps. Place over medium heat and cook, stirring frequently, until mixture begins to thicken (about 5-7 minutes).
Temper the egg yolks: Ladle about ½ cup of the hot milk mixture into the beaten egg yolks, whisking constantly. This prevents the eggs from scrambling. Pour the tempered egg mixture back into the saucepan and continue cooking over medium-low heat, stirring constantly, until the pudding thickens enough to coat the back of a spoon (about 2-3 minutes).
Finish the pudding: Remove from heat and stir in butter and vanilla extract until completely incorporated. Transfer to a bowl and press plastic wrap directly onto the surface of the pudding to prevent a skin from forming. Refrigerate until cooled but not completely set, about 30 minutes.
Prepare the whipped cream: In a chilled mixing bowl, beat the heavy cream, powdered sugar, and vanilla extract using an electric mixer until stiff peaks form. Be careful not to over-beat. Refrigerate until ready to use.
Slice the bananas: Peel and slice the bananas into approximately ¼-inch thick rounds. Toss gently with a few drops of lemon juice if not assembling immediately to prevent browning.
Begin assembly: Place a layer of vanilla wafers at the bottom of each parfait glass, breaking some cookies if necessary to create an even layer.
Add pudding and bananas: Spoon a layer of pudding (about 2-3 tablespoons) over the cookies, then arrange a layer of banana slices around the edge of the glass, pressing slightly into the pudding.
Continue layering: Repeat the layers—cookies, pudding, bananas—until you’ve reached near the top of each glass, ending with a layer of pudding.
Top with whipped cream: Add a generous dollop of whipped cream to each parfait, creating a decorative swirl with a spoon or piping bag if desired.
Garnish and chill: Sprinkle with crushed vanilla wafer crumbs if using, and refrigerate for at least 2 hours (or up to 8 hours) before serving to allow flavors to meld and cookies to soften slightly.