For the crumb crust: Preheat the oven to 180°C. Grease the sides of a 9-inch springform pan with butter. Wrap the bottom of the pan in foil to prevent leaking. Combine the sugar, salt, 8 of the gingersnap cookies and the digestives in a food processor and pulse until crumbly. Drizzle in the butter and continue pulsing until the mixture is the consistency of wet sand. Pour the mixture into the pan and push into the base and halfway up the sides using a glass with straight edges.
Bake the crust for about 10 minutes, and then let cool completely while you make the filling.
For the filling: Stir together the pumpkin, dark brown sugar, cinnamon, eggs, banana, egg white and milk in a large bowl. Pour the filling into the baked crumb crust and bake for about 50 minutes. Let cool completely while you make the ginger whipped cream.
For the ginger whipped cream: Whip the cream with the sugar until soft peaks form using a hand mixer. Fold in the ginger.
Run a thin knife around the edge of the pie to release it from the pan. Load the whipped cream into a piping bag with a large star tip. Pipe rosettes around the perimeter of the pie and decorate with the 8 remaining gingersnap cookies.
Cook's Note: Make sure you use a piping bag with a large star tip so the ginger bits in the whipped cream don't clog the tip.