Step 1

Preheat oven to 375 degrees with rack in upper third. Place a 10-inch cast-iron skillet (or other nonstick ovenproof pan) in oven. Whisk together flour, 3 tablespoons granulated sugar, baking powder, and salt in a bowl. Whisk together milk, egg, and melted butter in another bowl. Whisk milk mixture into flour mixture until just combined.

Step 2

Remove skillet from oven and add remaining 1 tablespoon butter, swirling to coat. Pour in batter and smooth top with an offset spatula. Sprinkle evenly with blueberries and remaining 1 teaspoon granulated sugar.

Step 3

Bake until golden brown and cooked through, about 25 minutes. Remove from oven; let cool 5 minutes before dusting with confectioners' sugar and serving with syrup.