1. Adjust oven rack to upper-middle position; heat oven to 350 degrees. Place two 12-ounce ramekins on aluminum foil-lined baking sheet and place 1 butter piece in each ramekin. Transfer to oven and heat until butter is melted, about 5 minutes.
2. Mash 1 tablespoon sugar and lemon zest with spoon. In separate bowl, mash together blueberries and 1 teaspoon sugar mixture until coarsely mashed.
3. Combine remaining sugar, flour, baking powder, and salt in large bowl. Whisk in milk and melted butter. Remove sheet pan from oven, transfer to wire rack, and divide batter evenly among ramekins.
4. Dollop blueberry mixture over batter and sprinkle with remaining lemon sugar. Bake 35 to 40 minutes, or until tops are brown and edges are crisp, rotating pan halfway through baking. Let cool on wire rack for 15 minutes. Serve warm.