Heat the oven to 350°F. Starting with a fully preheated oven helps the batter rise properly and keeps the fruit bubbling instead of steaming.
Cut the butter into pieces, place it in a 9x13 inch baking dish, and set the dish in the oven until the butter melts. Take the pan out as soon as the butter is melted so it stays rich and fragrant without browning too far.
Stir the blueberries, 1/2 cup granulated sugar, and lemon zest together in a bowl. The berries should look evenly coated and glossy, which helps the filling sweeten and soften evenly in the oven.
Whisk the flour, 1 cup granulated sugar, baking powder, and salt in a separate bowl, then stir in the milk just until a batter forms. The batter should be thick, smooth, and pourable, and stopping early keeps the topping tender instead of tough.
Spoon the blueberry mixture over the melted butter, then drizzle the batter over the top and finish with ground cinnamon. It may look rustic and uneven at this stage, but that loose layering is what gives cobbler its soft, craggy top.
Bake for about 35 to 40 minutes at 350°F. It is ready when the top is lightly golden and the blueberry filling is bubbling thickly around the edges.
Let the cobbler stand briefly before serving it warm. The resting time helps the filling thicken so it lands spoonable rather than runny.