FOR THE FILLING
5 cups raspberries
3/4 cup granulated sugar
2 tablespoons cornstarch
1/4 teaspoon coarse salt
FOR THE BISCUIT TOPPING
1 3/4 cups all-purpose flour
1 3/4 teaspoons baking powder
1/4 cup granulated sugar
1/4 teaspoon salt
6 tablespoons cold unsalted butter
3/4 cup heavy cream, plus more for brushing
Sanding sugar, for sprinkling (optional)
Make the filling: Preheat oven to 375 degrees. Mix together raspberries, granulated sugar, cornstarch, and salt. Pour into a 9-inch square baking dish (2 inches deep).
Make the biscuit topping: Whisk together flour, baking powder, granulated sugar, and salt. Cut in butter until largest pieces are the size of small peas. Add cream, and use a fork to incorporate, stirring just until cream is absorbed (there should be lots of loose pieces).
Turn out dough onto a clean surface, and knead once or twice, gathering loose bits into ball. Pat dough to a 1 1/4-inch thickness. Cut into 9 rough squares, and place them on top of filling. Brush with cream, and sprinkle with sanding sugar.
Bake cobbler until bubbling in center and biscuit topping is golden brown and cooked through, about 1 hour (loosely tent with foil if biscuit topping gets too dark). Let stand at least 30 minutes before serving.