Prepare the crust: In a food processor, crush the digestive biscuits or graham crackers until finely ground. Combine with melted butter in a bowl and stir until the mixture resembles wet sand. Press the mixture into the base of a 9-inch pie dish to form an even layer. Refrigerate the crust for at least 30 minutes to set.

Make the toffee filling: In a saucepan, combine sweetened condensed milk, butter, and dark brown sugar. Cook over medium heat, stirring constantly, until the mixture begins to boil. Once boiling, reduce the heat and simmer for 5-7 minutes, or until the toffee thickens. Stir continuously to prevent burning. Remove from heat and let it cool slightly.

Assemble the pie: Once the crust has set, pour the toffee filling over the chilled base. Smooth the surface with a spatula. Layer the banana slices evenly on top of the toffee.

Whip the cream: In a mixing bowl, beat the heavy cream, powdered sugar, and vanilla extract until soft peaks form. Spread the whipped cream over the bananas, covering them completely.

Garnish and chill: Optionally, garnish the pie with chocolate shavings or cocoa powder. Refrigerate for at least 2 hours, or overnight, to allow the pie to set fully.