• Preheat oven to400℉.
• Line a baking sheet with aluminum foil.

Prepare filling:

• Place blackberries, 1 cup sugar, lemon juice, and lemon zest in a mixing bowl.
• Dissolve cornstarch in cold water in a small bowl.  Add cornstarch slurry to the blackberry mixture. Stir the mixture together.
• Transfer blackberry mixture to a cast iron skillet and bring to a boil, stirring frequently.  Reduce heat and simmer until sauce thickens, about 5 minutes.  Remove from heat, set aside. If not using a cast iron pan transfer the filling to an oven safe dish.

Prepare topping:

• In a mixing bowl stir together flour, baking powder, 1/2 cup sugar, and salt.  
• Cut in cold butter using a pastry cutter or a fork until the mixture resembles coarse crumbs. Stir in the 1/4 cup boiling water and stir just until the mixture is evenly moist and crumbly.
• Top the blackberry mixture with the topping, dropping by spoonfuls.  Spread topping out towards the edges to cover the blackberry mixture in an even layer.
• Place pan on a foil lined baking sheet.  The filling will bubble while it bakes.  The baking sheet will catch any spills.
• Bake for 30 minutes or until the topping is a light golden brown.

To serve:

• Serve warm with vanilla ice cream.