Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Prepare the blueberry filling: In a large bowl, gently toss the blueberries with sugar, flour (or cornstarch), lemon juice, lemon zest (if using), and cinnamon. Pour this mixture evenly into the prepared baking dish.
Make the cobbler topping: In a separate medium bowl, whisk together the flour, sugar, baking powder, and salt. Add the cold, cubed butter and use a pastry blender, your fingertips, or a fork to cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
In a small bowl or liquid measuring cup, whisk together the milk and vanilla extract. Pour the wet ingredients into the dry ingredients and stir just until combined. Be careful not to overmix; a slightly lumpy batter is perfect.
Drop spoonfuls of the cobbler topping evenly over the blueberry filling. It doesn't need to cover perfectly; rustic patches are part of its charm.
Bake for 35-45 minutes, or until the topping is golden brown and the blueberry filling is bubbling around the edges. If the topping starts to brown too quickly, you can loosely tent the dish with aluminum foil.
Let the cobbler cool for at least 10-15 minutes before serving. This allows the filling to thicken slightly.