Prepare the Biscuit Base: First, crush your digestive biscuits. You can do this by placing them in a sealed plastic bag and crushing them with a rolling pin until they resemble fine crumbs, or use a food processor for a quicker job. Once crushed, pour them into a medium bowl. Melt the unsalted butter gently over a low heat or in the microwave until it's completely liquid and smells nutty. Pour the melted butter over the biscuit crumbs and stir thoroughly until every crumb is coated and the mixture feels uniformly damp and cohesive.

Assemble the Bases: Divide the buttery biscuit mixture evenly among six individual serving jars (jam jars or small tumblers work perfectly). Press the crumbs down firmly into the bottom of each jar using the back of a spoon or a small pestle. This creates a compact base that will hold its shape once chilled. The base should look even and slightly golden brown from the digestives. Pop these into the fridge to chill while you prepare the filling, allowing them to firm up slightly.

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 a whisk attachment, or an electric hand mixer. Beat on medium speed until the mixture is light, fluffy, and visibly smooth, with no lumps, taking about 2-3 minutes. The mixture should smell subtly sweet and creamy.

Add the Zesty Flavours: Now, it's time for the star ingredients! Add the fresh Key lime juice, Key lime zest, vanilla extract, and a tiny pinch of salt to the cream cheese mixture. If you're using condensed milk for an extra rich texture, add it now too. Mix again on low speed until everything is just combined. Be careful not to overmix at this stage; we just want to incorporate the liquids without deflating the cream cheese. The mixture should now have a distinct, bright citrus scent.

Fold in the Double Cream: In a separate, chilled bowl, whip the cold double cream until it forms soft peaks. It should look thick and luxurious, holding its shape when the whisk is lifted, but still somewhat fluid. Gently fold the whipped cream into the Key lime cream cheese mixture using a spatula. Use a light hand and fold just until no streaks of white cream remain, ensuring the filling remains airy and light. The final filling should be pale green/yellow and beautifully smooth.

Fill and Chill: Spoon or pipe the Key lime cheesecake filling evenly over the chilled biscuit bases in your jars. Smooth the tops with a spoon or the back of a knife. For a nice finishing touch, sprinkle a little extra Key lime zest over the top of each jar. Cover each jar loosely with cling film or a lid and refrigerate for a minimum of 4 hours, but ideally overnight. This chilling time is absolutely crucial for the cheesecake to set properly and develop its full, vibrant flavour. When ready, the cheesecake should feel firm to the touch and have a delightful wobble when gently nudged.