Follow these steps closely to achieve a perfect blueberry cobbler with a tender, cake-like topping and a bubbling, jammy fruit layer. The technique of pouring the batter over the melted butter without stirring is crucial for the final texture.

1. Preheat and Prepare the Pan: First, preheat your oven to 375°F (190°C). Place the 1/2 cup (1 stick) of unsalted butter into an 8×8-inch or 9×9-inch baking dish (or a similar-sized 2-quart dish). Place the dish inside the oven as it preheats. Allow the butter to melt completely and become hot. This step is vital for creating the signature crust. Once melted, carefully remove the dish from the oven and set it aside.
2. Prepare the Blueberry Filling: In a medium-sized bowl, combine the 4 cups of blueberries, 1/4 cup of granulated sugar, 1 tablespoon of lemon juice, and 1 tablespoon of cornstarch. Gently toss everything together until the blueberries are evenly coated. The cornstarch may look a little white and dusty, but it will dissolve as it bakes. Set the blueberry mixture aside for a moment.
3. Mix the Cobbler Batter: In a separate mixing bowl, prepare the dry ingredients for the topping. Whisk together the 1 cup of all-purpose flour, 3/4 cup of granulated sugar, 2 teaspoons of baking powder, and 1/4 teaspoon of salt. Whisking helps to evenly distribute the baking powder and salt, ensuring a consistent rise and flavor.
4. Combine Wet and Dry Ingredients: Pour the 3/4 cup of whole milk into the dry ingredients. Stir with a spatula or wooden spoon only until the ingredients are just combined. It’s very important not to over-mix the batter. A few small lumps are perfectly fine. Over-mixing can develop the gluten in the flour, resulting in a tough or dense topping instead of a light and tender one.
5. Assemble the Cobbler: Retrieve your baking dish with the hot, melted butter. Carefully pour the prepared cobbler batter directly over the layer of melted butter. Do your best to spread it evenly, but do not stir the batter and butter together. The layers must remain separate. Next, take your blueberry filling mixture and spoon it evenly over the top of the batter. Again, do not stir. It may seem counterintuitive, but during the baking process, the lighter batter will rise up through the heavier blueberries, creating a beautiful marbled effect with fruit on the bottom and a golden crust on top.
6. Bake to Golden Perfection: Place the assembled cobbler into the preheated 375°F (190°C) oven. Bake for 45 to 55 minutes. You’ll know it’s done when the fruit filling is hot and bubbly around the edges and the topping has risen and turned a deep golden brown. A toothpick inserted into the cakey part of the topping should come out with moist crumbs, but no wet batter.
7. Cool and Rest: This is a crucial, often-skipped step. Carefully remove the cobbler from the oven and place it on a wire cooling rack. Let it rest for at least 15-20 minutes before serving. This allows the molten-hot fruit filling to cool slightly and thicken up, preventing it from being too runny. This rest period ensures the cobbler holds its shape when served.