In a large mixing bowl, whisk together instant banana pudding mix and cold milk for about 2 minutes until thick. Let it chill for 5 minutes.
While the pudding sets, slice bananas (do this just before layering to prevent browning).
In a glass serving dish, layer Nilla wafers on the bottom.
Add a layer of sliced bananas on top of the wafers.
Spread half of the pudding mixture over the bananas.
Repeat the layers: wafers, bananas, then the rest of the pudding.
Top everything with Cool Whip, spreading evenly.
Crush a few extra wafers and sprinkle over the top.
Refrigerate for at least 2 hours (overnight is best) before serving.