For the blackberry sorbet, mix the blackberries with the sugar and cook for half an hour.
Then puree and pour through a fine-mesh sieve into a 22 x 22 cm mold. Mix in the buttermilk. Cover the mold and freeze for 8 hours.
Chop the frozen block, puree, return to the mold, cover and freeze repeatedly for 3 hours.
Garnish as desired.