You can substitute unthawed frozen blueberries for fresh berries, but increase the baking time in step 6 to 40 minutes.

Adjust oven rack to middle position and heat oven to 375 degrees. Line rimmed baking sheet with parchment paper.

FOR THE BISCUIT TOPPING: Whisk flour, 1 tablespoon sugar, baking powder, baking soda, and salt together in large bowl; set aside. Stir buttermilk and melted butter together in 2-cup liquid measuring cup (butter will clump; this is OK); set aside.

FOR THE FILLING: Combine sugar, cornstarch, lemon zest, and salt in large bowl. Add blueberries and lemon juice and mix gently with rubber spatula to combine. Transfer berry mixture to 8-inch square baking pan or ceramic dish. Place pan on prepared sheet and bake until filling is hot and starting to bubble around edges, about 25 minutes. Transfer sheet to wire rack and gently stir berry mixture. Increase oven temperature to 475 degrees.

Once oven reaches 475 degrees, add buttermilk mixture to flour mixture and stir with rubber spatula until just incorporated. Using greased ¼-cup dry measuring cup, drop 9 scant scoops of dough, evenly spaced, onto hot berry filling. Sprinkle tops with remaining 2 teaspoons sugar.

Bake until biscuits are golden brown and toothpick inserted in center biscuit comes out clean, 12 to 14 minutes. Melt remaining 1 tablespoon butter and brush over biscuits. Let cobbler cool on wire rack for at least 30 minutes. Serve.