Place the halved cherries along with the rest of the ingredients for the filling in a large saucepan. Place over medium heat and stir until no more flecks of cornstarch are visible.
Continue stirring frequently until the filling starts to bubble and thicken. Turn off the heat and transfer to a 9x13-inch (23x33 cm) baking dish. Evenly spread out the cherries and set aside.
For the biscuits, add the flour, granulated sugar, baking soda, baking powder, and salt to a large bowl. Whisk to combine.
Add the cold, cubed butter to the flour and use a pastry cutter to cut the butter into the flour until pea-sized clumps form.
Add the cold buttermilk then use a wooden spoon to mix it into the flour until a moistened, cohesive dough starts to form.
Carefully tip the dough out onto a lightly floured surface then quickly shape into a ball.
Sprinkle a bit of flour on your rolling pin and on top of the dough ball. Roll into a ¾-inch (2 cm) thick square.
Cut into 4 equal sized squares then stack them on top of each other. Sprinkle the top with a bit more flour then roll into a ½-inch (1.25 cm) thick slab.
Use a 2-inch (5 cm) round biscuit cutter to press out the biscuits (don't twist). Place them close to each other on top of the cherry filling. If your dough starts to get sticky, add a sprinkling of flour on top and on your hands. You should have about 18-20 mini biscuits.
Lightly beat an egg in a small bowl then brush the tops of the biscuits with the egg wash. Generously sprinkle the tops with 1 tbsp of coarse sugar.
Bake for 45 minutes or until the tops of the biscuits are golden brown and the filling is slowly bubbling along the edges.
Cool for 5-10 minutes then serve warm with a scoop of vanilla ice cream and an extra sprinkling of sumac.