Preheat Oven:
Preheat oven to 425°F (220°C). Grease a 9x9-inch baking dish.
Prepare the Filling:
In a medium bowl, combine blueberries, sugar, lemon juice, lemon zest, and cornstarch. Toss until evenly coated. Pour the mixture into the prepared baking dish.
Make the Topping:
In a separate bowl, mix flour, granulated sugar, brown sugar, baking powder, and salt. Cut in cold butter using a pastry cutter or fork until the mixture resembles coarse crumbs. Stir in boiling water (and vanilla if using) until just combined.
Assemble:
Drop spoonfuls of the topping over the blueberry filling. The topping doesn’t need to completely cover the berries.
Bake:
Bake for 25–30 minutes, or until the topping is golden brown and the filling is bubbly.
Cool and Serve:
Let the cobbler cool for at least 10 minutes before serving. Serve warm with vanilla ice cream or whipped cream, if desired.