Make the biscuit topping: whisk together flour, sugar, salt, and baking powder. Add cold butter and pinch into the flour with your fingers until shaggy and crumbly. Add buttermilk and mix until just combined. Pop the dough in the fridge or freezer while you make the filling. This keeps the butter cold and solid, which makes it easier to work with and gives you a better texture.
Make the filling: toss blueberries, sugar, lemon zest, and cornstarch in a bowl. Pour into a greased 2-quart baking dish.
Crumble the dough over the blueberry filling in rough, uneven pieces. (For a 2 qt dish, use about ⅔ of the dough.)
Place the baking dish on a sheet pan to catch bubbling juices.
Bake 55-60 minutes (add 5-10 min for frozen berries), until the topping is deep golden and filling is bubbling at the edges.
Cool at least 20 minutes before serving. Serve warm or cold with vanilla ice cream, whipped cream, or Coco Whip.
Notes
Lemon: Use zest only (or skip) for a thicker filling when serving chilled. Swap for lemon juice for a brighter, saucier filling.
Frozen berries: No need to thaw. Add 5-10 minutes to bake time.
Storage: Cover and refrigerate up to 4 days. Reheat at 300°F; leave uncovered the last few minutes to re-crisp the topping.
Freeze: Baked cobbler freezes well up to 2-3 months. Thaw overnight in the fridge and reheat in the oven.