Whip the cream very stiff and chill. Mix the buttermilk with the powdered sugar until smooth. Soak the gelatine in cold water.
Squeeze the limes, pass the juice through a sieve and then heat gently. Gently squeeze out the soaked gelatine and dissolve it in the slightly heated lime juice. Then mix with the buttermilk.
Now fold the cold cream into the buttermilk mixture, fill into 4 smaller bowls or 1 larger bowl and refrigerate immediately for 8 - 9 hours (but better overnight).
This goes well with red fruit sauce or red fruit sauce (pureed from any red fruit).