Make the biscuit base: Place the digestive biscuits in a sealed plastic bag and crush them with a rolling pin until you have fine crumbs. You should hear a consistent crunching sound as they break down. Alternatively, pulse them in a food processor until they resemble fine sand. In a bowl, mix the crumbs with the melted butter and caster sugar until the mixture looks like wet sand and holds together when pressed between your fingers.

Press the crust into the tin: Tip the crumb mixture into a 20cm loose-bottomed tart tin or pie dish. Use the bottom of a glass or a flat measuring cup to press it down firmly and evenly across the base and up the sides. The crust should feel compact and solid, not crumbly. Place it in the fridge to set while you make the filling.

Beat the cream cheese: In a large bowl, beat the softened cream cheese with a hand mixer or stand mixer fitted with the paddle attachment until it is completely smooth and fluffy. You’ll notice the colour lighten slightly and the texture become airy, with no lumps remaining.

Add the condensed milk and lime: Pour in the sweetened condensed milk, key lime juice, key lime zest, and vanilla extract. Mix on low speed until everything is just combined. The mixture will thicken and become a pale, creamy yellow as the lime juice reacts with the condensed milk. Do not overmix, or the filling may become too thin.

Whip the cream: In a separate, clean bowl, whip the double cream and icing sugar together until soft peaks form. You’re looking for the cream to hold its shape when the whisk is lifted, but it should still be soft and billowy, not stiff or grainy.

Fold and fill: Gently fold the whipped cream into the cream cheese and lime mixture using a spatula. Fold until no white streaks remain; the filling should be uniform in colour and light, mousse-like texture. Pour the filling into the chilled biscuit base and spread it out evenly with the spatula. The top should look smooth and glossy.

Chill to perfection: Cover the pie loosely with cling film and refrigerate for at least 4 hours, but preferably overnight. The filling needs this time to set completely; it should feel firm to the touch and slice cleanly when ready. For the final touch, garnish with extra lime zest just before serving.