In a large bowl, stir together the cornstarch an 1/4 cup cold water until cornstarch is completely dissolved. Add 1 cup sugar, lemon juice, and blackberries, and combine the mixture gently but thoroughly. Transfer to an 8-inch cast-iron skillet. In a bowl, combine well the flour, remaining sugar, baking powder, and salt. Blend in the butter until the mixture resembles coarse meal. Add 1/4 cup boiling water and stir the mixture until it just forms a dough. Bring the blackberry mixture to a boil on top of the stove, stirring. Drop spoonfuls of the dough carefully onto the boiling mixture, and bake the cobbler on a foil lined baking sheet in the middle of a preheated 400f oven for 20-25 minutes or until the topping is golden. Serve warm with vanilla ice cream.
A 1989 Gourmet Mag. favorite