Heat the oven to 160°C (140°C fan). While it’s heating, place your biscuits in a sealed plastic bag and crush them with a rolling pin until you have fine crumbs that feel like damp sand when you rub them between your fingers. Alternatively, blitz them in a food processor until they’re evenly ground — you want no large chunks left.

Mix the crust. Tip the biscuit crumbs into a bowl, add the melted butter and caster sugar, and stir with a fork until every crumb is coated. The mixture should clump together when you squeeze a handful. Press this firmly into a 23cm pie dish, using the flat bottom of a glass to compact the base and up the sides evenly. Bake for 10 minutes, until the crust looks slightly darker and smells toasty and buttery.

Make the filling while the crust cools. In a medium bowl, whisk the egg yolks for about 30 seconds until they’re pale and slightly thickened — they should look like runny custard. Pour in the condensed milk and whisk until completely smooth, about 1 minute. The mixture will be thick and glossy.

Add the lime juice and zest. Slowly pour in the lime juice while whisking continuously. You’ll notice the mixture instantly thicken — this is the acid reacting with the condensed milk. Keep whisking until it’s uniform and has the texture of a thick milkshake. Stir in the lime zest and a pinch of salt. Taste a tiny dab — it should be sharply tangy with a creamy finish.

Pour and bake. Pour the filling into the warm (not hot) crust. Spread it evenly with a spatula. Bake for 15 minutes — the centre should still jiggle slightly when you gently shake the dish, like a wobble in jelly. The edges will look set and slightly puffed. Don’t overbake or the pie can crack.

Cool completely. Place the pie dish on a wire rack and let it cool to room temperature. This takes about 45 minutes. You’ll see the filling settle and become firmer as it cools. Once it’s no longer warm to the touch, cover loosely with cling film and refrigerate for at least 3 hours, but overnight is ideal.

Whip the cream (optional but heavenly). Just before serving, pour the double cream into a chilled bowl with the icing sugar and whisk until soft peaks form — the cream should hold its shape when you lift the whisk but still look soft and billowy. Spread or pipe over the chilled pie, then finish with extra lime zest.