Remove the label from the can of sweetened condensed milk and put it in a big pot of water. Bring the water to a boil and then reduce to a simmer for 3 hours until the milk caramelizes. Check the pot regularly to make sure there’s always plenty of water in the pot, enough that the can is always completely covered.
While the milk is “cooking,” cut the banana into 3/4” thick slices and cover the bottom of the graham cracker crust.
Once the milk is done, open the can and pour the caramelized milk over the layer of bananas. Top the milk layer with Cool Whip or freshly whipped cream.
Refrigerate for one hour. Slice and serve.