Make the biscuit base: Place the digestive biscuits in a sealed bag and crush them with a rolling pin until you have fine crumbs with no large chunks — you should hear a consistent crunch, not loud cracks. Alternatively, pulse them in a food processor. Transfer the crumbs to a medium bowl, add the melted butter and golden caster sugar, then stir until the mixture resembles damp sand that clumps when pressed between your fingers.

Press the crust: Tip the crumb mixture into a 23cm loose-bottomed tart tin or pie dish. Using the flat base of a measuring cup or a glass, press the crumbs firmly into the base and up the sides. The crust should feel compact and solid, with no loose crumbs shifting when you tilt the dish. Place the dish in the refrigerator while you prepare the filling.

Whisk the egg yolks: In a large mixing bowl, whisk the egg yolks until they become pale and slightly thickened — this takes about two minutes with a hand mixer. The colour should shift from deep orange to a lighter, creamier yellow, and the mixture should fall from the whisk in a thick ribbon.

Add the condensed milk: Pour in the sweetened condensed milk and whisk on low speed until fully combined. The mixture will feel heavy and smooth, almost like a thick custard. Scrape down the sides of the bowl to ensure everything is incorporated evenly.

Incorporate the lime juice and zest: With the mixer running on low, slowly pour in the key lime juice. The mixture will thicken almost immediately as the acid reacts with the condensed milk — you will see it transform from a runny liquid to a pudding-like consistency within seconds. Fold in the lime zest by hand, then taste the filling; it should be tangy but not puckeringly sour.

Pour and chill: Remove the crust from the refrigerator. Pour the filling into the chilled crust and spread it evenly with a spatula. The surface should be smooth and level. Gently tap the dish on the counter twice to release any air bubbles. Cover loosely with cling film and refrigerate for at least 4 hours, or overnight for a firmer set.

Whip the cream topping: Just before serving, pour the double cream, icing sugar, and vanilla extract into a chilled bowl. Whip until soft peaks form — the cream should hold its shape when you lift the whisk, but still look luscious and slightly droopy. Over-whipping will turn it grainy, so stop as soon as you see those soft peaks.

Assemble and serve: Spread or pipe the whipped cream over the chilled pie. Grate a little extra lime zest over the top for colour. Slice with a sharp knife dipped in hot water and wiped dry for clean cuts. Each slice should reveal a firm, pale yellow filling against the golden crust.