01 - Preheat oven to 375°F (190°C) and lightly grease a 9-inch square baking dish.
02 - Combine blueberries, sugar, cornstarch, lemon zest, and lemon juice in a bowl. Toss gently to coat evenly.
03 - Pour the blueberry mixture into the prepared baking dish and spread in an even layer.
04 - Whisk together flour, sugar, baking powder, and salt in a medium bowl.
05 - Add chilled, cubed butter to flour mixture. Use a pastry blender or fingertips to cut in butter until mixture resembles coarse crumbs.
06 - In a small bowl, whisk milk and egg together. Stir milk mixture into flour mixture just until combined; do not overmix.
07 - Drop spoonfuls of topping evenly over blueberry filling, leaving some gaps for juices to bubble through.
08 - Bake for 35-40 minutes, or until topping is golden and filling is bubbling around the edges.
09 - Allow cobbler to cool slightly before serving warm, optionally with a scoop of vanilla ice cream.