Prepare the Crust: Preheat your oven to 180°C (160°C fan). Place the digestive biscuits in a food processor and blitz until they form fine crumbs. Alternatively, put them in a sealed bag and bash with a rolling pin. Transfer the crumbs to a bowl, add the caster sugar and salt, and pour in the melted butter. Mix with a fork until the mixture resembles wet sand.

Bake the Crust: Tip the crumb mixture into a 23cm (9-inch) pie dish. Press the crumbs firmly and evenly across the bottom and up the sides of the dish. I find that using the back of a measuring cup helps to get a really compact and even base. Bake for 10 minutes until it’s fragrant and slightly golden. Set aside to cool completely.

Start the Filling: While the crust cools, make the filling. Place the finely chopped dark chocolate in a heatproof bowl. In a small saucepan, gently heat 150ml of the double cream until it just begins to simmer around the edges – don't let it boil. Immediately pour the hot cream over the chopped chocolate and let it sit for 2-3 minutes without stirring.

Create the Ganache Base: After a few minutes, gently stir the chocolate and cream mixture. Start from the centre and work your way out in small circles. The chocolate will melt into the cream, creating a smooth, glossy ganache. Stir in the vanilla extract. In a separate small bowl, whisk the egg yolk lightly and then slowly whisk it into the slightly cooled chocolate mixture.

Whip the Cream: In a large, separate bowl, use an electric mixer to whip the remaining 150ml of double cream with the sifted icing sugar and cocoa powder. Whip until you have soft peaks. Be careful not to over-whip.

Combine and Chill: Gently fold about a third of the whipped cream mixture into the chocolate ganache to lighten it. Then, carefully fold in the remaining whipped cream until just combined and there are no streaks left. Pour this beautiful, airy filling into the cooled biscuit crust and smooth the top with a spatula. Place in the refrigerator to chill for at least 4 hours, or preferably overnight, until firm.

Make the Frosting: Once the pie is fully set, make the frosting. In a large bowl, beat the room-temperature butter with an electric mixer for 3-4 minutes until it's very pale and creamy. Add the sifted icing sugar and cocoa powder in two batches, mixing on a low speed to combine, then increasing the speed to beat until smooth. Add the vanilla and 2 tablespoons of milk, and beat on high speed for 3-5 minutes until the frosting is light, fluffy, and spreadable. Add the final tablespoon of milk only if needed to reach your desired consistency.

Frost the Pie: You can either spread the frosting evenly over the top of the chilled pie with an offset spatula for a rustic look, or transfer it to a piping bag fitted with a star nozzle to create decorative swirls. I love piping big, generous rosettes around the edge. Serve chilled.