Preheat the oven to 350 degrees F. In a food processor, combine the graham crackers and sugar. Pulse until the graham crackers are fine crumbs.
Pour in the crumbs into a medium mixing bowl, stir in the butter and whisk until incorporated.
Pour the mixture into a 9½ inch pie dish. Use a flat measuring cup to press the mixture into the pie dish. Make sure it's packed tightly.
Bake for 7 minutes. Cool at room-temperature for 30 minutes, then cool in the fridge for another 40 minutes.
For the filling:
In a 2-quart saucepan, melt the butter over medium-heat. Add the brown sugar and whisk until dissolved. Whisk in the sweetened condensed milk, then boil for 2-3 minutes, whisking continuously. Pour the filling into the chilled pie dish. Cover and cool in the fridge for at least one hour.
For the topping & to assemble:
Take the pie out of the fridge. Place the banana slices in a single layer on top of the toffee filling.
Combine the heavy cream, sugar, and vanilla extract in a large mixing bowl. With an electric mixer, whip the cream on high-speed until the cream reaches stiff peaks, about 2-3 minutes. Spread the whipped cream over the pie. Sprinkle the grated chocolate over the pie, then serve. Keep the pie chilled when not serving.
Recipe by Sprig and Flours at http://www.sprigandflours.com/banoffee-pie/