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 with a few slightly larger bits for texture. Alternatively, pulse them in a food processor until you hear a consistent, soft crunching sound. Pour the crumbs into a medium bowl, add the melted butter, and stir with a fork until every crumb looks damp and darkens slightly, clumping together when pressed between your fingers.

Press the crust: Tip the buttery crumbs into a 20cm (8-inch) pie dish. Using the flat bottom of a glass or a measuring cup, press the mixture firmly and evenly across the base and up the sides. The surface should feel compact and smooth, with no loose crumbs shifting. Place the dish in the freezer for 10 minutes while you make the filling.

Mix the filling base: In a large mixing bowl, whisk the sweetened condensed milk, double cream, and vanilla extract together until the mixture looks pale, glossy, and slightly thickened, about 1 minute of vigorous whisking. You should see soft ribbons form when you lift the whisk.

Add the lemon: Pour in the lemon juice and add the zest and salt. Stir gently but continuously for about 30 seconds. The mixture will immediately begin to thicken as the acid reacts with the condensed milk — you'll feel the resistance change from a liquid to a soft, spoonable consistency. Stop as soon as it looks uniform and creamy, with no streaks of juice visible.

Assemble the pie: Remove the chilled crust from the freezer. Spoon the filling into the crust, spreading it evenly with a spatula right to the edges. The surface should be smooth and level, with a slight wobble when you gently shake the dish.

Freeze: Place the pie on a level shelf in the freezer, uncovered, for at least 4 hours. After the first 2 hours, the surface will feel firm to a gentle touch and no longer sticky. Once fully set, cover with cling film or foil and freeze for up to 1 week.

Serve: Remove the pie from the freezer about 5-8 minutes before slicing. Run a sharp knife under hot water, wipe it dry, then slice cleanly through. The first cut should reveal a bright, pale yellow interior with a crisp, golden crust edge. Serve immediately while still firm but scoopable.