Rinse the fresh blackberries and toss them in a bowl with the first portion of sugar, lemon juice, and cornstarch. This resting period is a top-notch professional move that allows the sugar to draw out the natural juices.
In a medium bowl, combine the flour, the second portion of sugar, baking powder, salt, and cinnamon. Whisk these dry ingredients thoroughly to ensure the leavening agent is distributed.
Melt the unsalted butter and pour it directly into a 9×13 inch baking dish. Tilt the pan so the butter coats the bottom and sides completely.
Whisk the milk and vanilla extract together, then pour them into your dry flour mixture. Stir with a wooden spoon until just combined and still slightly lumpy. Avoiding over-mixing is a top-notch baker’s secret; it keeps the gluten relaxed so the final crust has a soft, cake-like texture rather than becoming tough and rubbery.
Pour the batter over the melted butter in the baking dish, but do not stir them together. Carefully spoon the sugared blackberries over the top of the batter. As the dish bakes, the dough will rise up around the fruit.
Slide the dish into an oven preheated to 375°F (190°C) and bake for 45 to 55 minutes. You are looking for a deep golden-brown crust and thick, bubbling purple juices. Let the cobbler cool on a wire rack for at least 20 minutes before serving.
