Preheat and prep: Heat the oven to 350°F (175°C). Place the stick of butter in a 9x13-inch baking dish and set it in the oven to melt while you mix the batter. Keep an eye on it so it doesn’t brown unless you prefer a toasty flavor.

Sweeten the berries: In a large bowl, combine blueberries with 1/3 to 1/2 cup sugar, the lemon zest, and 1–2 teaspoons lemon juice. If your berries are very juicy, toss in 1 tablespoon cornstarch. Add a pinch of cinnamon or nutmeg if you like. Set aside.

Whisk the dry ingredients: In another bowl, whisk flour, 2 teaspoons baking powder, 1/2 teaspoon salt, and 1/2 cup sugar. Adjust sugar to taste if you like a sweeter topping.

Make the batter: Stir in the milk and vanilla until just combined. Don’t overmix. The batter should be pourable and smooth, without lumps of dry flour.

Layer the dish: Carefully remove the hot baking dish from the oven. Tilt to coat the bottom evenly with melted butter. Pour the batter directly into the melted butter. Do not stir. Spoon the blueberries and their juices evenly over the batter. Again, don’t mix; the batter will rise up around the fruit as it bakes.

Bake: Place the dish on the center rack and bake for 40–50 minutes, until the top is golden and set, and the berries are bubbling around the edges. If the top browns too quickly, tent loosely with foil for the last 10 minutes.

Rest and serve: Let the cobbler cool for at least 15–20 minutes. It thickens as it rests. Serve warm with vanilla ice cream, whipped cream, or a drizzle of heavy cream.