Pre-heat the oven to 180°C.

Mix the biscuits and butter together thoroughly and press into a cake tin. Leave in the fridge to harden while you do everything else.

Whisk the egg yolks until light and fluffy, then add the lime zest, the juice and the condensed milk. Whisk a little more to loosen and combine, then pour this on top of the biscuit.

Whisk the egg whites until soft peaks form, then gradually add the sugar, vanilla and cream of tartar. Continue whisking until it reaches firm peaks.

Spoon the egg white on top of the pie and use a spoon or fork to raise ribbons on the surface. Chuck in the oven and bake for 20 minutes or until the top is golden brown.

Remove from the oven, allow to cool and refrigerate until needed - the flavours are much better cold.