Make the biscuit base: Place the digestive biscuits in a sealed plastic bag and crush them with a rolling pin until they resemble coarse sand—you want a few tiny lumps for texture, but no large shards. Tip the crumbs into a bowl, pour in the melted butter, and stir with a fork until every crumb is dampened and the mixture smells warmly of toasted biscuit. Press this firmly into the base and up the sides of a 23cm pie dish, using the flat bottom of a glass to compact it evenly. Pop it in the fridge to firm up while you prepare the filling.

Whisk the dry ingredients: In a medium saucepan, combine the caster sugar, cornflour, cocoa powder, and salt. Whisk them together thoroughly—you shouldn't see any white streaks of cornflour against the cocoa. This ensures a lump-free filling later.

Temper the egg yolks: In a separate bowl, whisk the egg yolks lightly. Pour the milk into the saucepan with the dry ingredients and whisk until smooth, then place the pan over a medium heat. Cook, whisking constantly, until the mixture thickens and begins to bubble at the edges—you'll hear a soft plopping sound as the first bubbles break the surface. At this point, cook for exactly one more minute, whisking all the while; the mixture will have the glossy, thick texture of a pudding and smell richly of cooked chocolate.

Finish the filling: Remove the pan from the heat. Immediately add the chopped dark chocolate and the vanilla extract. Stir gently but continuously until the chocolate is fully melted and the mixture is uniformly dark, smooth, and glossy. The residual heat is enough to melt the chocolate without scrambling the eggs.

Assemble the pie: Pour the hot filling into the chilled biscuit base. Use a spatula to spread it evenly to the edges. The surface should look like a dark, still pond. Place a piece of cling film directly onto the surface of the filling (this prevents a skin from forming) and refrigerate for at least 4 hours, until the filling is firm and slightly springy to the touch.

Whip the cream: Just before serving, pour the double cream and icing sugar into a chilled bowl. Whisk with an electric mixer on medium speed until soft peaks form—when you lift the whisk, the cream should hold a gentle, drooping peak that doesn't collapse. Be careful not to overwhip, or you'll end up with butter.

Finish and serve: Spread or pipe the whipped cream over the chilled chocolate filling. Scatter with chocolate shavings or curls if you like. Slice with a sharp knife dipped in hot water and wiped dry between cuts for the cleanest edges.