Pour the sweetened condensed milk into a baking dish. Cover with aluminum foil and set in a larger baking dish. Fill the larger dish with hot water until it reaches halfway up the sides of the smaller dish.
Place in the oven and cook for 90 minutes. Check the water level occasionally, making sure that it reaches halfway and refilling as necessary.
Carefully remove the baking dish from the hot water bath and let cool to room temperature. Whisk the mixture (now it’s dulce de leche!) until smooth.
Reduce the oven temperature to 350˚F (180˚C).
In the bowl of a food processor, combine the graham crackers and melted butter. Pulse until crackers are finely ground and the mixture resembles wet sand.
Grease a 9-inch (22 cm) tart pan with nonstick spray. Add the graham cracker mixture to the tin and press evenly to cover pan completely.
Bake crust for 5 minutes, until golden and fragrant. Transfer to a wire rack and let cool to room temperature.
Spread the dulce de leche in the cooled tart crust, using a flexible spatula to smooth it into an even layer. Cover and refrigerate for at least 4 hours or up to overnight, until set.
In a large bowl, beat the heavy cream until soft peaks are just beginning to form. Add the sugar and beat until soft peaks form.
Remove the pie from the refrigerator. Scatter the banana slices over the dulce de leche. Top with the whipped cream, spreading evenly to cover bananas, and garnish with chocolate shavings.