Place the caster sugar in a bowl, then
rub in the vanilla seeds with your fingers so
they’re evenly mixed. Stir in the yogurt until
the sugar has dissolved. Churn the mixture
in an ice-cream machine until frozen but still
soft or, if you don't have a machine, pour the
yogurt into a freezer-safe container and freeze
for 4-6 hrs, stirring thoroughly every hr or so.
Meanwhile, line a 1kg loaf tin with cling film.
Spoon the soft frozen yogurt into the tin,
cover with another piece of cling film, then
freeze for at least 4 hrs, until firm. Segment
the grapefruit, catching the juices in a bowl
– you should get about 200ml. Put the juice
into a small pan with the honey, simmer for
10-15 mins until thickened and syrupy, stir
in the segments, then leave to cool.
Take the yogurt ice from the freezer about
10 mins before you want to serve it. Cut into
slices and serve topped with grapefruit, some
of the honeyed sauce and a scattering of mint.