In a medium saucepan off the heat, whisk together the sugar, cornstarch, and salt. Add 0.5 cup of the milk and whisk until smooth, then whisk in the egg yolks until fully combined. This prevents lumps from forming.

Whisk in the remaining 2 cups milk. Place over medium heat and cook, whisking constantly, until the mixture thickens and large bubbles break the surface, about 6 to 8 minutes. The pudding should coat the back of a spoon and hold a line when you run your finger through it.

Remove from heat and immediately whisk in the cold butter and vanilla until smooth. Press through a fine-mesh sieve into a bowl to remove any cooked egg bits. Press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate until cold, at least 2 hours or up to 3 days.

When ready to assemble, slice the bananas into 0.25-inch rounds. Beat the cold heavy cream and powdered sugar with a whisk or electric mixer until soft peaks form, about 2 minutes. Do not overbeat or it will become grainy.

Place 3 vanilla wafers in the bottom of each cup. Add a layer of pudding (about 0.25 cup), then a layer of banana slices. Repeat with 2 more wafers, another layer of pudding, and more bananas. Top with a final layer of pudding and 1 wafer standing upright for garnish.

Dollop or pipe the whipped cream on top of each cup. Refrigerate uncovered for at least 30 minutes and up to 4 hours before serving. The wafers should soften slightly but still have some texture.