Make the biscuit base: Place the digestive biscuits in a sealed plastic bag and crush them with a rolling pin until they resemble fine breadcrumbs — you want a uniform, sandy texture with no large chunks. Tip the crumbs into a bowl, pour in the melted butter, and stir until every crumb is coated and the mixture looks like wet sand. Press this firmly into the base and up the sides of a 23cm pie dish. Use the bottom of a glass to get an even, compact layer. The base should feel solid and hold together when pressed. Pop it in the fridge to set while you make the filling.

Whisk the dry ingredients: In a medium saucepan, whisk together the caster sugar, cocoa powder, cornflour, and salt. Make sure there are no lumps of cocoa or cornflour — the mixture should look like a smooth, dark powder. The colour will be a deep, rich brown, and it will smell faintly of chocolate.

Add the milk and egg yolks: Slowly pour in the milk while whisking continuously. You want a smooth, lump-free paste. Then add the egg yolks and whisk again until fully incorporated. The mixture will be thin and milky at this stage, a pale brown colour with a slight sheen.

Cook the filling: Place the saucepan over medium heat and stir constantly with a wooden spoon or a whisk. Don't walk away — this is the critical part. After about 3-4 minutes, you'll feel the mixture start to thicken. It will go from a thin liquid to a thick, glossy pudding in a matter of seconds. You'll hear it start to bubble gently, and it will look like a dark, shiny chocolate sauce. Let it bubble for exactly 1 minute while stirring — this cooks the cornflour and ensures a silky texture. The final consistency should be thick enough to coat the back of a spoon heavily, and it will smell intensely of chocolate.

Finish the filling: Remove the saucepan from the heat and stir in the vanilla extract. The filling will be very thick and glossy, with a smooth, pudding-like texture. Pour it immediately into the prepared biscuit base. Use a spatula to spread it evenly. The surface should be smooth and level. Tap the dish gently on the counter to release any air bubbles.

Cool and set: Let the pie cool at room temperature for about 20 minutes — you'll see the filling firm up slightly as it cools. Then transfer it to the fridge and chill for at least 4 hours, or overnight. The filling should be completely set, firm to the touch but still soft and silky when you press it. The surface will look matte and smooth.

Whip the topping: Just before serving, pour the double cream into a chilled bowl. Add the icing sugar if you want a slightly sweeter topping. Whip with a hand mixer or stand mixer until soft peaks form — the cream should hold its shape but still look soft and billowy. You'll know it's ready when you lift the beaters and the cream forms gentle peaks that flop over slightly. Spread or pipe the cream over the chilled pie. Sprinkle with chocolate shavings if you like.