Heat caramel either in a large glass bowl in the microwave for about 2 minutes, or in a saucepan over low medium heat, stirring frequently, until soft. Remove from heat and whisk in egg yolks, vanilla and lime zest and juice until smooth (the residual heat will lightly cook the yolks). Leave to cool to room temperature.
Meanwhile, place egg whites in a clean, dry mixing bowl and whip with an electric beater on high until foamy. Add caster sugar and continue whipping until you have a thick meringue. Set aside.
Whip cream in another bowl until soft peaks have formed (I don’t bother washing the beater in between beating the egg whites and cream). Be careful not to over-whip the cream.
Add one-third of the meringue to the cooled caramel mixture and fold in using a large metal spoon (keeping it as airy as possible), then fold in about a third of the cream, doing the same, to lighten the mixture. Lightly fold through remaining meringue and whipped cream. Spoon or ladle into serving glasses and chill in the fridge for at least 3 hours.
When ready to serve, top mousse with diced mango, peaches, canned tropical fruit, passionfruit sauce – whatever takes your fancy.