Using hand-held electric beaters, beat mascarpone, sugar and vanilla in a large bowl until smooth and fluffy. Beat in the cream on low speed. Set aside

Halve, remove stones and cut 4 nectarines into 1.5cm-thick wedges. Place into a bowl. Drizzle with lime juice and sprinkle with 2 teaspoons caster sugar. Toss to combine. Combine caramel and salt in a small bowl. Place coffee into a shallow dish

To assemble the tiramisu, dip half of the biscuits into the coffee for about 1 second on each side and use to line the base of an 8-cup (about 7cm deep) dish, trimming biscuits as required. Top with half of the mascarpone and swirl with half of the salted caramel. Drain nectarines and arrange in an even layer on the mascarpone mixture

Repeat layering using the remaining biscuits, coffee and mascarpone mixture. Drizzle with remaining salted caramel. Cover and refrigerate for 6 hours

Just before serving, halve, remove stones and cut the remaining 2 nectarines into thin wedges. Arrange nectarines on the tiramisu and serve