How to Make It
Preheat oven to 350°.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, soda, and salt, stirring with a whisk.
Place granulated sugar and butter in a large bowl; beat with a mixer at medium speed until well blended (about 2 minutes). Add vanilla, egg, and egg white; beat well. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture; mix after each addition.
Spoon half of the batter into a 9-inch round baking pan coated with cooking spray. Sprinkle evenly with 1 cup blueberries. Spoon remaining batter over the blueberries; sprinkle evenly with remaining 1 cup blueberries. Sprinkle the top evenly with 1 tablespoon turbinado sugar. Bake at 350° for 50 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on wire rack.
Note: If using peak-season fruit, use 1 1/2 cups blueberries instead of 2 cups, and only 1 cup buttermilk instead of 1 1/3 cups. This will make the batter thicker so the berries won't sink to the bottom.