This is Felicity Cloake’s “perfect” version of this southern US speciality.
Heat the oven to 160C (150 fan)/325F/gas 3 and grease a 20cm tart tin. Crush the biscuits to a fine crumb (this is easiest in a food processor, if you have one).
Stir in the melted butter or oil and salt, tip into the tin and press down firmly into the base and around the sides.
Bake for 15 minutes, until dry to the touch, then remove and leave to cool before making the filling.
Finely grate the zest from all four limes into a large bowl, add the egg yolks, then squeeze the juice from the fruit into a jug until you have 120ml.
Beat the yolks and zest for a couple of minutes, until the yolks thicken and turn a pale green, then stir in the condensed milk.
Finally, beat in the lime juice and taste – it should be shockingly tangy and sweet, though you can add more juice if you like, bearing in mind that chilling will make the flavour more subtle.
Pour the filling into the cooled biscuit shell and chill for at least three hours, until set. Just before serving, whisk the cream and sugar in a large bowl until the mix holds soft peaks, then dollop on top. Garnish with extra lime slices, if you have them.