Heat oven to 350℉.
Spray a 12 inch cast iron skillet with baking spray. Set aside.
In a large bowl, toss blueberries with 4 Tablespoons sugar and 4 Tablespoons flour. Set aside.
In another large bowl, whisk 2½ cups flour, 5 Tablespoons sugar, lemon zest, baking powder, and salt together.
Add eggs, egg yolk, melted butter, milk.
Whisk until combined. (It doesn't have to be smooth.)
Pour into prepared pan.
Spread blueberries on top of batter.
Bake for 65-70 minutes, or until the top is golden brown and the cobbler is cooked through.