Preheat oven to 325°F. Grease a 9-inch springform pan and line the bottom with parchment. Wrap the outside with a double layer of foil to guard against leaks.
Make the crust: Stir wafer crumbs, 0.25 cup sugar, and 0.25 tsp salt. Add melted butter and mix until evenly moistened. Press firmly into the bottom and slightly up the sides of the pan. Bake 10 minutes; cool while you make the filling.
Beat cream cheese on medium speed until smooth and fluffy, about 2 minutes. Add 0.75 cup sugar, cornstarch, and 0.25 tsp salt; mix just to combine, scraping the bowl.
Mix in sour cream, mashed banana, lemon juice, and 2 tsp vanilla until smooth. Beat in eggs one at a time on low, just until incorporated (do not overmix).
Pour batter over the crust and smooth the top. Bake at 325°F for 50 to 55 minutes, until edges are set and the center still has a slight jiggle.
Turn off the oven, crack the door, and let the cheesecake cool inside for 1 hour. Remove, then chill uncovered until cold, at least 6 hours or overnight.
Make banana cream topping: Whisk cold milk and pudding mix for 2 minutes; let thicken 5 minutes. Whip heavy cream, powdered sugar, and 0.5 tsp vanilla to soft peaks. Fold most of the whipped cream into the pudding until light and smooth.
Spread the banana cream over the chilled cheesecake. Top with banana slices and crushed wafer cookies. Pipe or dollop the remaining whipped cream around the edges. Slice and serve.
Notes
For neat slices, dip a thin knife in hot water and wipe dry between cuts. Toss banana slices with a few drops of lemon juice to slow browning. Store covered in the refrigerator for up to 3 days.