Mix the crust: In a bowl, combine the graham cracker crumbs and melted butter. Stir until every crumb is coated and the mixture feels like wet sand. Set aside. The mixture should hold together when pressed.
Make the mousse: Using a handheld electric mixer, beat the softened cream cheese and powdered sugar together until smooth and creamy — about 2 minutes. Beat in the sweetened condensed milk, key lime juice, granulated sugar, and lime zest until creamy and well incorporated. The mixture should be thick and smooth, with no lumps. Taste it here — you should get a bright, tart pop followed by creamy sweetness.
Chill the mousse (essential for clean layers): Place the mixture in the fridge for at least 1 hour, ideally 2, to firm up. This makes it much easier to pipe without the layers bleeding together. I never skip this anymore — it's the difference between a pristine parfait and a muddy one.
Prep your piping bag: Transfer the chilled mousse to a piping bag fitted with a 1/2-inch tip. If you don't have a piping bag, a sturdy zip-top bag with one corner snipped off works like a charm — just don't overfill it. The mousse should feel dense and creamy, not runny.
Layer the parfaits: Alternate the graham cracker crumbs and the cheesecake mousse into the 8 jars, starting with the crackers and ending with the mousse. I do about 2 tablespoons of crumbs, then a generous layer of mousse, then repeat. Each jar gets about 1/4 cup of crumbs total and about 1/2 cup of mousse. Tap the jar gently after each layer to settle it.
Secure lids and chill: Put the lids on the jars and place them in the fridge for at least 3 hours to set completely. Overnight is even better — the flavors meld and the texture firms up beautifully. The wait is torture, but worth it.
Top and serve: Unfasten the lids, pipe some whipped cream on top, and garnish with a sprinkle of key lime zest. Serve cold and watch them disappear. The first spoonful should be a contrast of crumbly crust and silky mousse.