The recipe isn't clear on the amount of sugar, saying 1/2 cup in the main custard ingredient list then referencing 1/4 cup in the instructions. I used 1/2 cup & it seemed right.
From EatingWell: May/June 1997
Buttermilk gives this silky panna cotta, or “cooked cream,” a bright, fresh taste. A tart-sweet strawberry sauce is a nice contrast.