Prepare the filling. Combine the sugar and cornstarch in a mixing bowl. Add the berries, lemon juice, and zest and stir. Let sit while preparing the dough.
For the dough, whisk together the flour, 3 tablespoons sugar, baking powder, baking soda, and salt. Add the butter and, using a pastry cutter or your fingers, work the butter into the flour until it resembles a coarse meal with some bigger pieces of butter. Add the buttermilk and stir until combined.
Pour the filling into a 10-inch cast iron skillet. Drop tablespoonfuls of the dough on top of the berries. Sprinkle the dough with 1 tablespoon sugar. Bake for 40 – 50 minutes or until the dough is golden brown and the berries are bubbling. Let cool a little before serving.