Make the Biscuit Base: Place the digestive biscuits in a food processor and blitz into fine crumbs. Alternatively, put them in a sealed bag and bash with a rolling pin. Transfer to a bowl, stir in the caster sugar, then pour in the melted butter and mix until it resembles wet sand. Press this mixture firmly and evenly into the bottom and up the sides of a 23cm (9-inch) pie dish. Place in the fridge to chill while you make the filling.

Melt the Chocolate: Place the chopped dark chocolate in a heatproof bowl over a saucepan of barely simmering water (a bain-marie), ensuring the bottom of the bowl doesn't touch the water. Stir occasionally until smooth and completely melted. Remove from the heat and set aside to cool slightly for about 10 minutes.

Heat the Eggs and Sugar: In a separate large, clean heatproof bowl, whisk together the eggs and caster sugar. Set this bowl over the same saucepan of simmering water. Whisk continuously for 5-7 minutes, until the sugar has completely dissolved and the mixture is warm to the touch. For complete safety, use a food thermometer to ensure the mixture reaches 71°C (160°F), as recommended by the Food Standards Agency. This step gently cooks the eggs.

Whip the Egg Mixture: Remove the bowl from the heat. Using an electric hand mixer or a stand mixer with the whisk attachment, beat the egg mixture on high speed for 8-10 minutes. It will become very pale, thick, and triple in volume. The key is to continue whipping until the mixture has completely cooled to room temperature.

Combine with Chocolate: Gently fold the slightly cooled melted chocolate and the vanilla extract into the whipped egg mixture until just combined. Be careful not to knock out too much air.

Prepare the Butter: In the bowl of your stand mixer (or a separate large bowl), beat the softened butter on medium-high speed for 2-3 minutes until it's very light, pale, and fluffy.

Create the Silk Filling: With the mixer on a low speed, add the chocolate-egg mixture to the whipped butter one large spoonful at a time. I find this gradual addition is what works best for me to prevent the mixture from splitting. Wait until each spoonful is fully incorporated before adding the next. Once it's all combined, increase the speed to medium-high and beat for a final 2 minutes until the filling is glossy and smooth.

Chill the Pie: Pour the finished chocolate filling into your chilled biscuit base and smooth the top with a spatula. Cover loosely with cling film and place in the fridge to set for at least 4 hours, or preferably 6 hours or overnight.

Decorate and Serve: Just before serving, whip the cold double cream with the icing sugar until soft peaks form. Spoon or pipe the whipped cream over the top of the set pie. Use a vegetable peeler to shave curls from a bar of dark chocolate and sprinkle them over the cream.