To make the dulce de leche, place the sweetened condensed milk in a double boiler (a heatproof bowl set on top of a saucepan with an inch of simmer water) for about 3 hours, until thick and butterscotch colored. You may need to refill the water once or twice. Let cool.
Preheat the oven to 350 degrees F. For the crust, take the gingersnap cookies for a spin in the food processor to grind it into a fine crumb.
Mix together the ground gingersnaps, salt, cinnamon, and melted butter, and press it out into a pie plate. Bake the crust for 10 minutes, then cool completely on a wire rack.
Pour the dulce de leche into the crust. Slice the bananas, then lay them on the dulce de leche.
Place the heavy cream, sugar, and vanilla extract in a bowl and whip to soft peaks. Spread on top of the bananas, then dust the top with cocoa powder or extra dulce de leche, and serve. Enjoy!