Make the crust. Stir graham crumbs, melted butter, sugar, and a pinch of salt until the mixture looks like wet sand.
Portion the base. Spoon 1–2 tablespoons of crust into each cup. Press gently with the back of a spoon.
Set aside.
Whip the cream. In a cold bowl, beat heavy cream and powdered sugar to medium-stiff peaks. Do not overbeat. Chill the bowl while you make the filling.
Blend the cheesecake base. Beat softened cream cheese until smooth.
Add sweetened condensed milk, lime juice, lime zest, vanilla, and a pinch of salt. Mix until silky and lump-free.
Fold it together. Gently fold the whipped cream into the cheesecake base in two additions until just combined. Keep it airy.
Fill the cups. Spoon or pipe the mixture over the crusts, leaving a little room at the top for garnish.
Chill to set. Cover and refrigerate for at least 2 hours, or up to overnight, until firm and mousse-like.
Garnish and serve. Add a sprinkle of lime zest, a tiny slice of lime, or extra graham crumbs.
Serve cold.