Heat half & half with 1 cup of sugar.
Temper in egg yolks, cook until mixture coats the back of a spoon.
Strain and let cool.
Heat blackberries with sugar until just scalded.
Purée and strain.
Mix purée and cream into custard, add vanilla, salt, lemon juice and kirsch.
Put in the refrigerator (not the freezer) until it is cold.
Chill a mason jar in a bowl of ice in the freezer.
Pour the cold mixture into the mason jar and mix slowly with a long-handled wooden spoon, until it starts to set and forms a thick, creamy texture.
Put the lid on the mason jar and put it in the freezer.