Prepare the Biscuit Base: Crush the digestive biscuits until they resemble fine crumbs. You can do this by placing them in a sealed bag and crushing with a rolling pin, or by pulsing in a food processor until you hear a consistent whirring and see a fine, sandy texture. Melt the unsalted butter gently in a small saucepan over a low heat until completely liquid and shimmering. Pour the melted butter over the biscuit crumbs and mix thoroughly until all the crumbs are evenly coated, taking on a slightly darker, moist appearance.

Assemble the Bases: Divide the buttered biscuit crumbs evenly among 6-8 small jars or glasses. Press the crumbs down firmly into the bottom of each jar using the back of a spoon or a small tamper. This step is crucial for a stable base; you should feel a slight resistance and hear a compacting sound as you press. Place the jars in the refrigerator while you prepare the filling.

Whip the Cream Cheese Mixture: In a large mixing bowl, beat the softened full-fat cream cheese with the caster sugar using a stand mixer fitted with the whisk attachment, or a hand mixer, on medium speed. Continue mixing until the mixture is light, fluffy, and visibly smooth, with no lumps, typically taking about 2-3 minutes. Scrape down the sides of the bowl to ensure everything is incorporated.

Add Key Lime Flavour: Gradually beat in the key lime juice, key lime zest, and vanilla extract into the cream cheese mixture. Mix on low speed until just combined. The mixture will smell wonderfully zesty and should have a slightly looser consistency. Be careful not to overmix at this stage, as it can make the filling too thin.

Whip the Double Cream: In a separate, chilled bowl, whip the cold double cream until stiff peaks form. The cream should hold its shape when you lift the whisk and look visibly thicker and opaque. Be attentive to the sound of your mixer; it will change from a liquid slosh to a thicker, whirring sound as the cream thickens.

Combine and Fill: Gently fold the whipped double cream into the key lime cream cheese mixture using a spatula. Use a light hand, folding until no streaks of white cream are visible and the mixture is uniformly pale green-yellow and airy. Spoon or pipe the cheesecake filling into the prepared jars over the biscuit base. Fill them nearly to the top, ensuring a smooth surface.

Chill and Set: Cover each jar loosely with cling film or a lid and refrigerate for a minimum of 2 hours, or preferably overnight. As they chill, the cheesecake will firm up and become beautifully set. When ready to serve, the surface should feel firm to the touch and appear glossy.