Step 1: Make the Biscuit Base

• Crush the biscuits: Use a food processor or place the digestive biscuits in a plastic bag and crush them with a rolling pin until fine crumbs form.
• Mix with butter: Combine the crushed biscuits with the melted butter in a bowl and stir until well combined.
• Press into the tin: Transfer the biscuit mixture into the base of a loose-bottomed tart tin and press it down firmly with the back of a spoon to create an even layer. Chill the base in the fridge for 30 minutes to set.

Step 2: Prepare the Key Lime Filling

• Mix the filling: In a mixing bowl, whisk together the key lime juice, sweetened condensed milk, and egg yolks until smooth and well combined.
• Add lime zest: Stir in the zest of one lime to add extra citrus flavor.
• Pour into the base: Once the base has chilled, pour the lime filling over the biscuit base and smooth the top with a spatula.

Step 3: Bake the Pie

• Preheat the oven: Set the oven to 160°C (140°C fan)/325°F/gas mark 3.
• Bake the pie: Place the pie in the oven and bake for 15 minutes, or until the filling is set but still has a slight wobble in the center.
• Cool completely: Remove the pie from the oven and allow it to cool completely at room temperature before chilling in the fridge for at least 2 hours.

Step 4: Add the Whipped Cream Topping

• Whip the cream: In a mixing bowl, whisk the double cream until soft peaks form.
• Top the pie: Spread or pipe the whipped cream on top of the chilled pie.
• Garnish: Sprinkle the pie with lime zest for a decorative finish.