Preheat oven to 375 degrees F.

Put the blueberries in a large bowl.

In a small bowl, combine the granulated sugar and lemon zest. Rub together with your fingers until fragrant. Add the sugar lemon mixture to the blueberries. Add the brown sugar and corn starch. Gently stir until blueberries are well coated. Let the mixture sit while you prepare the cobbler topping.

In a medium bowl, whisk together the flour, baking powder, salt, and sugar. Cut the cold butter into the flour mixture with a pastry blender, two forks, or by using clean hands. Mix until the butter pieces are pea size.

In a small bowl, whisk the cold buttermilk and vanilla together, then pour mixture into dry ingredients. Stir with a spatula until mixture comes together. Don't overmix, the dough will be sticky.

Pour the blueberry mixture into a 9" x 13" pan. Drop pieces of the cobbler topping on top of the blueberries, it is fine if there is space between the cobbler topping. Lightly brush the cobbler topping with heavy cream or milk. Sprinkle turbinado sugar over the cobbler topping.

Place in the oven and bake for 45-50 minutes or until the cobbler topping is golden brown and the blueberries are bubbling. Remove from the oven and let cool on wire rack for 15 minutes before serving. Serve with vanilla ice cream, if desired.