Preheat oven to 325°F. Mix crushed wafers, sugar, and melted butter. Press into a 9-inch springform pan and bake for 10 minutes. Let cool.
Beat cream cheese until smooth. Add sugar, then eggs one at a time. Mix in vanilla, mashed bananas, sour cream, and cornstarch.
Wrap pan in foil. Pour filling over crust. Place in a water bath and bake for 60–70 minutes until slightly jiggly. Cool in the oven for 1 hour, then chill at least 6 hours.
Whisk pudding mix with cold milk. Chill until thick. Whip cream with sugar and vanilla, then fold into pudding. Spread on chilled cheesecake.
Top with banana slices, whipped cream, and crushed wafers before serving. Use a warm knife for clean slices.