Make the biscuit crust: Place the digestive biscuits in a sealed bag and crush them with a rolling pin until they resemble fine, sandy crumbs. You want a uniform texture with no large shards. Tip the crumbs into a bowl, stir in the 30g of caster sugar, then pour over the melted butter. Mix until every crumb is coated and the mixture clumps together when pressed between your fingers.

Press and bake the crust: Tip the buttery crumbs into a 23cm loose-bottomed tart tin. Press them firmly and evenly across the base and up the sides using the flat bottom of a glass or a measuring cup. The layer should feel compact and solid, not crumbly. Bake at 170°C (fan) for 10 minutes — you'll smell a warm, toasty, biscuity aroma. Allow it to cool completely on a wire rack while you make the filling.

Whisk the dry base: In a medium saucepan, whisk together the 100g caster sugar and the cornflour until no white lumps remain. Add the egg yolks one at a time, whisking vigorously after each addition. The mixture will be thick and paste-like, and it should turn a pale, lemony yellow colour.

Heat the milk and combine: In a separate small saucepan, warm the 500ml of whole milk over a medium heat until it's steaming and small bubbles form around the edge — do not let it boil. Remove from the heat. Slowly pour the hot milk into the egg yolk mixture in a thin, steady stream, whisking constantly. This gentle, gradual addition prevents the eggs from scrambling. The mixture will look thin and milky at this point.

Cook the custard: Return the mixture to the saucepan and place it over a medium-low heat. Stir continuously with a wooden spoon or a heatproof spatula, scraping the bottom and sides. After 3 to 4 minutes, you'll feel the mixture begin to resist your stirring — it will thicken visibly, and you'll hear a soft bubbling sound. Once it's as thick as a pudding and holds a clear trail when you drag the spoon through, remove it from the heat. This takes roughly 5 to 7 minutes total.

Add the chocolate and butter: Immediately tip the finely chopped dark chocolate into the hot custard and stir until it has melted completely. The mixture will turn a deep, glossy mahogany colour. Add the cubed butter and the vanilla extract, stirring until the butter is incorporated and the filling looks smooth and shiny. Pour the hot filling into the cooled biscuit crust, spreading it evenly with a small offset spatula.

Chill the pie: Press a piece of cling film directly onto the surface of the filling — this prevents a skin from forming. Refrigerate for a minimum of 4 hours, though overnight is better. The filling should be completely set and feel firm to a gentle touch, with no wobble in the centre.

Whip the cream and finish: Just before serving, pour the double cream and icing sugar into a chilled bowl. Whisk with an electric mixer until soft peaks form — the cream should hold its shape when you lift the whisk, but it should not look grainy or stiff. Spoon or pipe the cream over the set chocolate filling. Finish with chocolate shavings or curls and a light dusting of cocoa powder for a professional, finished look.