In a large bowl, whisk together the milk and instant banana pudding. Add 8 oz of the Cool Whip and whisk again until smooth.
Layer 1/3 of the graham crackers in the bottom of a square casserole dish. About 4 rectangles in each layer. You'll have to break some to make them fit.
Spread half of the pudding mixture on top. Then top evenly with sliced bananas.
Layer another 1/3 of the graham crackers. Then spread the other half of the pudding on top. Then top evenly with more sliced bananas.
Layer the remaining 1/3 of the graham crackers on top. And then evenly spread the remaining 4 oz of Cool Whip to cover the graham crackers.
Cover the dish with plastic wrap. Refrigerate for 8 hours or overnight.
When ready to serve, sprinkle the top evenly with the chopped peanuts (if using). Then top with more sliced bananas.