- Juice and zest the limes. Bring the zest, cream and vanilla bean to a rolling boil. Boil for exactly 4 minutes until the cream separates. Add the lime juice and citric acid, then return to the boil. Lower the heat and simmer gently for exactly 1 minute, then remove from the heat.
- Pour the cream through a fine sieve into a stainless-steel bowl, then refrigerate until set. Rinse the vanilla bean clean under cold running water.
- Line a clean sieve with damp muslin and place it over a larger bowl. Pour in the set cream, cover with plastic wrap, and return to the fridge for 24 hours. Scoop the mascarpone into a clean container and refrigerate. It will keep for 7-10 days.