Firstly, to make the crust roughly crush the digestive biscuits and pop them in the food processor and blitz until finely ground. Gradually pour in the melted butter while the processor is still blitzing. Press this mixture into the base and along the side of the pie dish, using the back of a table spoon to flatten the mixture evenly.
Bake in the oven for about 20 minutes until it is firm to the touch and deep golden in colour. Leave aside to cool completely.
Turn the oven temperature down to 150°C, Gas 2. Toss the egg yolks, condensed milk, lime juice and zest in a bowl and lightly whisk until well combined. This mixture naturally thickens so don’t worry if it’s a little thin.
Pour the filling on top of the base and bake in the preheated oven for 20-30 minutes. The filling should be firm, not wobbly but slightly soft in the centre. Allow to cool completely before placing in the fridge for at least an hour.
Whip the cream before you serve and spread over the pie. Decorate with a little grated zest or a slice of lime.