Preparation

• Heat the oven to 350 degrees F and coat an 8x8-inch baking dish lightly with grease. The prepared dish should be ready before the fruit mixture is assembled.
• Combine the blueberries, sugar, and salt in a large bowl. Whisk the water and cornstarch separately until smooth, then fold the slurry through the berries. The fruit should look evenly coated, with no visible dry cornstarch.
• Spread the berry mixture in the baking dish and bake fresh berries for 18 minutes or frozen berries for 30 minutes. Put the dish on a baking sheet if its sides are shallow. The filling should begin bubbling, and the baking sheet will catch sugary juices if they rise over the edge.
• Remove the dish and stir from the center outward, scraping thickened fruit from the bottom and sides back into the mixture. Return it to the oven for another 15 minutes. After this second fruit bake, the juices should look thicker and bubble steadily rather than remaining watery.
• While the fruit finishes, sift the flour, sugar, baking powder, cinnamon, and salt in a large mixing bowl. The dry mixture should have an even color, with the cinnamon distributed throughout.
• Pour in the milk, melted butter, and vanilla, then stir just until the batter comes together. Stop when the mixture is smooth and thick enough to flow slowly from a spoon.
• Pour the topping over the hot fruit as soon as the second fruit bake is complete, spreading it gently if needed. Return the dish to the oven for 38 to 45 minutes. The crust is ready when its center passes a toothpick test and the edges have turned golden.
• Remove the sonker from the oven and serve it warm with vanilla ice cream. The berry corners should remain soft and jammy while the topping holds a tender golden slice.