In a large, covered pot, boil can of unopened (with wrapper removed) sweetened condensed milk for 4 hours (yep, you read that right - 4 hours).
The can must be completely submerged in water the entire time.
Allow the can to cool completely before use.
For the whipped cream topping:
In a medium bowl, with an electric mixer, whisk heavy cream, powdered sugar and vanilla together until stiff peaks form.
To assemble:
Once can of sweetened condensed milk can (toffee) has cooled, spread the toffee filling into the graham cracker crust.
Slice bananas and layer on top of the filling.
Spread whipped cream on top of bananas and sprinkle with chocolate shavings or chocolate sprinkles. Keep pie refrigerated.
Video
Notes
This is a pretty rich dessert so serve in relatively small slices
COOL WHIP can be substituted for instead of the homemade whipped cream.
Sweetened condensed milk can be cooked in a slow cooker if you don't want to do it on the stovetop. Place can in a crock pot (a smaller crock pot works great but any size will do). Cover can with hot tap water in the slow cooker until the water is two inches above the top of the can (you can lay the can on its side. Set slow cooker for 8 hours on low. Remove can from crock pot using tongs. Allow the can to cool completely before opening. You can put the can into some cold water to speed up the process.