Make the crumb crust: Put graham cracker crumbs to in a bowl and stir in melted butter. Press into a 9"-diameter tart base with a removable bottom. Press the mixture up the sides of the tart form with the back of a spoon. Chill the crust in refrigerator for at least one hour.

Make the toffee: Melt the butter in a small saucepan. Stir in the brown sugar and melt over low heat. Add the condensed milk and bring the mixture to a boil for a few minutes, stirring continuously. The toffee should darken slightly. Pour the filling into the crust. Cool and chill again for at least one hour until the caramel is firm.

Remove the tart from the pan and carefully transfer to a serving plate. Slice the bananas and place them in a single layer on top of the caramel. Whip the heavy cream and spoon it over the toffee and bananas, sealing the filling in. Sprinkle the top of the pie with grated chocolate.