Shiny Top Blackberry Cobbler

Ingredients

• 5 cups fresh or frozen blackberries
• 1 1/2 tablespoons lemon juice
• 2 cups flour
• 3 cups sugar, divided
• 1 cup milk
• 1/3 cup butter or margarine, softened, cut into small pieces.
• 2 teaspoons baking powder
• 1 teaspoon salt, divided
• 2 tablespoons cornstarch
• 1 1/2 cups boiling water