If you don't have access to black raspberries, you can substitute blackberries, red raspberries, or just add an extra cup of cherries.
5 cups sweet cherries, stemmed, pitted
1 cup black raspberries
1/4 cup cornstarch
1/2 cup granulated sugar, plus 1 tablespoon for topping
1 teaspoon vanilla extract
Juice from 1/2 small lemon
1/4 teaspoon salt
Pinch ground cinnamon
2 cups flour
2 teaspoons baking powder
1/2 teaspoon salt
12 tablespoons cold unsalted butter, cut into cubes
1/2 cup milk, plus 1 teaspoon for topping
Preheat the oven to 375°F. Butter a 9-inch deep-dish pie plate.
Combine the pitted cherries, black raspberries, cornstarch, sugar, vanilla, lemon juice, salt, and cinnamon in a large bowl. Stir well.
Add the flour, baking powder, and salt to the bowl of a food processor and pulse to combine. Add the butter and pulse until the mixture forms a coarse meal, 15 to 30 seconds depending on the size of your cubes. Dump the mixture into a medium bowl and add the milk. Mix with a fork until a scrappy dough forms. (If you don't have a food processor, you can cut the butter into the dry ingredients in a mixing bowl with a pastry blender, fork and knife, or your fingers, and then add the milk).
Turn the dough onto the counter and knead three or four times, folding the dough over on itself, until it comes together. If the dough seems too dry, drizzle a tiny bit more milk over the dry areas and knead again once or twice. Press the dough into a disk about 1/2-inch thick. With a biscuit cutter or a jelly jar, cut straight down into 10 or 12 shapes, enough to cover the cobbler (recombine the scraps to make extra biscuits). Arrange the biscuits on top, brush with a little milk, and sprinkle with the remaining sugar.
Bake uncovered until the biscuits are golden brown, 40 to 45 minutes. Let cool at least 10 minutes. Serve with vanilla ice cream.