Prepare the biscuit base: Place the digestive biscuits in a sealed food bag and crush them with a rolling pin until you have fine, sandy crumbs – you should hear the crackle of the biscuits breaking down, and the crumbs should feel even and dry. Transfer to a bowl, pour in the melted butter and caster sugar, and stir with a fork until the mixture resembles damp, dark sand that clumps together when pressed.

Form the crust: Tip the buttery crumbs into a 23cm (9-inch) loose-bottomed tart tin. Press them firmly and evenly into the base and up the sides using the flat bottom of a glass or a measuring cup. The crust should feel compact and solid, not crumbly. Bake at 160°C (320°F) for 10 minutes, until the aroma of toasted biscuits fills the kitchen. Set aside to cool completely.

Make the filling – the crucial moment: In a large bowl, whisk the egg yolks until they are thick and pale – this should take about 2 minutes by hand, and the mixture will look like a thick, glossy ribbon falling from the whisk. Pour in the sweetened condensed milk and whisk again until fully combined and smooth.

Add the lime: Slowly pour in the lime juice while whisking continuously. Watch as the mixture instantly begins to thicken and lighten in colour, a reaction between the acid and the condensed milk. The texture will become noticeably heavier and creamier. Fold in the lime zest until evenly distributed. The filling should smell intensely of fresh lime.

Assemble and bake: Pour the filling into the cooled biscuit crust. Gently tap the tin on the countertop to release any air bubbles. Bake at 160°C (320°F) for 14-16 minutes. The centre should still have a slight wobble when gently shaken, and the edges will be just set. The surface will look matt, not shiny or wet.

The essential cool-down: Remove from the oven and let the pie cool completely on a wire rack. You will feel the warmth slowly dissipate from the tin. Once at room temperature, cover loosely with cling film and refrigerate for a minimum of 4 hours, ideally overnight. The filling will become firm, sliceable, and wonderfully dense.

Whip the cream: Just before serving, pour the double cream, icing sugar, and vanilla extract into a chilled bowl. Whisk until soft peaks form – the cream should hold its shape but still feel luscious and billowy, not stiff. Spread or pipe the cream over the chilled pie, then garnish with a generous shower of fresh lime zest.