For the Biscuit Base:

1. Prepare the Tin: Grease a 20cm (8-inch) loose-bottomed tart tin and line the base with parchment paper.
2. Make the Biscuit Base: Crush the digestive biscuits into fine crumbs and mix them with the melted butter until well combined.
3. Press into the Tin: Press the biscuit mixture evenly into the base and up the sides of the prepared tart tin. Use the back of a spoon to smooth it out. Place the tin in the refrigerator to chill while you prepare the filling.

For the Lime Filling:

4. Preheat the Oven: Preheat your oven to 160°C (140°C fan) or 325°F.
5. Prepare the Filling: In a mixing bowl, whisk the egg yolks until slightly thickened. Gradually whisk in the sweetened condensed milk until smooth.
6. Add Lime Zest and Juice: Stir in the finely grated lime zest and the freshly squeezed lime juice. Mix until fully combined and the filling has thickened slightly.
7. Fill the Tart Shell: Pour the lime filling into the chilled biscuit base, spreading it evenly with a spatula.
8. Bake the Pie: Bake in the preheated oven for 15 minutes, or until the filling is just set and has a slight wobble in the center.
9. Cool and Chill: Remove the pie from the oven and allow it to cool to room temperature. Once cooled, refrigerate the pie for at least 2 hours, or until fully set.

For the Topping:

10. Whip the Cream: In a mixing bowl, whip the double cream with the icing sugar until soft peaks form.
11. Top the Pie: Spread the whipped cream over the chilled pie, creating swirls with the back of a spoon. Garnish with extra lime zest if desired.
12. Serve: Carefully remove the pie from the tart tin, slice, and serve chilled.