Crush the Digestive Biscuits finely (use a food processor or place in a bag and bash with a rolling pin). Transfer the crumbs to a bowl and stir in the melted Unsalted Butter until the mixture resembles wet sand. Press the mixture firmly into the base and up the sides of a 9-inch (23 cm) loose-bottomed tin to form an even crust, then chill in the fridge for 20–30 minutes to set.
While the base chills, make the toffee: melt a knob of the Unsalted Butter in a medium saucepan over low heat, add the Light Brown Sugar and stir until dissolved. Pour in the Canned Sweetened Condensed Milk and continue to cook gently, stirring constantly, until the mixture thickens and turns a deep caramel color (about 10–15 minutes). Be careful to keep the heat low and stir to prevent sticking or burning.
Remove the caramel from the heat and let it cool slightly, then pour it evenly over the chilled biscuit base. Smooth the surface with a spatula and return the tin to the fridge to set for at least 30 minutes.
Slice the Banana into 1/4-inch (6 mm) rounds and arrange the slices in a single layer over the set caramel.
Whip the Double Cream with the Vanilla Extract until soft peaks form — you want a light, billowy texture rather than an overly stiff cream. Spoon or pipe the whipped cream over the bananas to cover them completely.
Finish by grating or shaving the Dark Chocolate over the top for a bitter-chocolate contrast, then chill the pie for another 30 minutes to an hour to allow the layers to settle.
When ready to serve, run a knife around the tin edge, remove the pie, slice and enjoy. Keep leftovers refrigerated and consume within 2–3 days.
