Preheat the oven to 350 degrees F.
Zest one lemon, adding the zest to the sugar.
zest of one lemon, 1-1/2 cups granulated sugar
In a medium sized bowl, stir together blackberries and 1/3 cup sugar until well coated. Set them aside to marinate for 30 minutes.
4-1/2 cups (12 oz.) fresh or frozen blackberries
In another bowl, whisk together flour, baking powder, salt and remaining sugar. Make a well in the center of the flour mixture and stir in milk and vanilla until fully incorporated.
1-1/2 cups all-purpose flour, 1 tbsp. baking powder, 1 tsp. table salt, 1-1/2 cups whole milk, 1 tsp. vanilla extract
Add butter to a 10-inch cast iron skillet. Place in the preheated oven until butter is fully melted (4-5 minutes).
12 tbsps. unsalted butter
Remove skillet from the oven and pour the batter over the melted butter without stirring. Sprinkle blackberry mixture evenly over the batter. (It's optional if you want to sprinkle another teaspoon of sugar on top of the batter. It really depends on how sweet your blackberries are to start with.)
1 tsp. granulated sugar
Bake the cobbler 45-50 minutes, until bobbly, puffed and golden brown. An inserted toothpick will come out mostly clean.
Let it cool on a wire rack for at least 30 minutes before serving. Serve with vanilla ice cream.