Adjust oven rack to middle position and heat oven to 400 degrees. Spray 12-cup muffin tin with vegetable oil spray.
Whisk flour, cornmeal, baking powder, baking soda, and salt together in medium bowl. Whisk eggs in second medium bowl until well combined and light colored, about 20 seconds. Add sugar to eggs and whisk vigorously until thick and homogeneous, about 30 seconds. Add melted butter in 3 additions, whisking to combine after each addition. Add 6 tablespoons sour cream and 1/4 cup milk and whisk to combine, then whisk in remaining 6 tablespoons sour cream and remaining 1/4 cup milk until combined.
Add wet ingredients to dry ingredients and mix gently with rubber spatula until batter is just combined and evenly moistened. (Do not over mix) Using ice cream scoop or large spoon, divide batter evenly among prepared muffin cups, dropping batter to form mounds. (Do not level or flatten batter)
Bake until muffins are light golden brown and toothpick inserted in center of muffin comes out clean, about 18 minutes, rotating muffin tin halfway through baking. Let muffins cool in muffin tin for 5 minutes, then transfer to wire rack, let cool 5 minutes longer, and serve warm.