Prepare the crust. Place the digestive biscuits in a sealed plastic bag and crush them with a rolling pin until they resemble fine, sandy crumbs. You want a uniform texture with no large chunks. Tip the crumbs into a bowl and stir in the melted butter and caster sugar until the mixture feels like damp sand that clumps when pressed between your fingers.

Press and bake. Tip the crumb mixture into a 23cm pie dish. Use the flat bottom of a glass or a measuring cup to press it firmly and evenly across the base and up the sides. The crust should feel compact and smooth under your fingers. Bake at 160°C (fan) for 10 minutes, until the fragrance of toasted biscuit fills your kitchen and the edges are just beginning to deepen in colour. Set aside to cool completely.

Whisk the filling. In a large bowl, whisk the egg yolks until they are thick and pale, a process that takes about two minutes. The colour will shift from bright yellow to a soft, buttery lemon. Pour in the sweetened condensed milk and whisk until the mixture is smooth and glossy, with a texture like thick custard.

Add the lime. Slowly pour in the Key lime juice while whisking constantly. You will see the mixture begin to thicken almost immediately as the acid reacts with the condensed milk. The aroma is sharp and fragrant. Fold in the grated lime zest until it is evenly distributed, then pour the filling into the cooled crust.

Bake the pie. Place the pie in the oven at 160°C (fan) for 12 to 15 minutes. You are looking for the edges to be set and slightly puffed, while the centre should still have a slight wobble when you gently shake the dish. The surface will look matte, not wet. Remove from the oven and let it cool to room temperature on a wire rack.

Chill thoroughly. Once the pie is at room temperature, cover it loosely with cling film and refrigerate for at least 2 hours, but preferably 4 hours or overnight. The filling will become firm and sliceable, with a clean, cool texture. The wait is essential for the authentic texture.

Whip the topping. Just before serving, pour the chilled double cream into a bowl with the icing sugar and vanilla extract. Whisk by hand or with an electric mixer until soft peaks form. The cream should hold its shape but still look luscious and pillowy. Spread or pipe the cream over the chilled pie, then garnish with extra lime zest.