Place the digestive biscuits or graham crackers into a food processor and blend until you get a fine crumb. Pour in the butter and blend again to mix. Tip into a 9-inch pie dish and press down firmly to cover the base and sides of the dish. Chill for 30 minutes.
Spread the dulce de leche into the chilled crust in an even layer. Cut the bananas into coins about ¼-inch thick and cover the dulce de leche with them.
Place the heavy cream into a medium bowl and beat with a whisk until thickened. Spread over the sliced bananas. Use a peeler to peel shavings of dark chocolate over the whipped cream. Chill for at least an hour before serving. Keep covered in the fridge for up to 3 days.