Heat the oven to 375°F (190°C). Butter the dish and put it on a rimmed sheet. Check pitted cherries carefully for missed stones.
Toss cherries with fruit sugar, cornstarch, and lemon juice. Spread evenly in the dish, including any coating left in the mixing bowl.
Whisk topping flour, sugar, baking powder, and salt. Cut in cold butter until small flakes and pea-size pieces remain.
Add buttermilk and stir with a fork until a rough dough forms. Avoid kneading it smooth, which can toughen the biscuits.
Divide dough into eight rough mounds over the fruit. Leave spaces between them so the filling can bubble and the biscuit edges can brown.
Bake 35–45 minutes, until biscuits are golden and a tester in the center of a biscuit shows no raw dough.
Check that fruit juices bubble near the middle. If topping browns too quickly, tent loosely with foil and finish baking.
Let stand 20 minutes before serving. Spoon a biscuit and some thickened cherry filling into each bowl, taking care with the hot fruit juices.
