Preheat the oven to 400 degrees F (200 degrees C). Prepare a standard 12-cup muffin tin with liners.
For cornflake topping: add cornflakes, sugar, and melted butter to a bowl. Stir to combine. Place in the freezer while making the batter.
For muffin batter, combine melted butter, sugar, salt, and vanilla in a mixing bowl. Add egg and whisk to combine.
Combine flour, baking powder, and baking soda in a small bowl.
Add half the flour mixture. Stir until mostly incorporated, then add sour cream and buttermilk. Add remaining flour mixture and stir until combined.
Toss blueberries with 2 teaspoons flour in a small bowl. Fold berries into the batter.
Divide the batter evenly between the muffin cups.
Top each muffin with a generous amount of cornflake mixture (you can break it up some when you pull it out of the freezer).
Bake for 10 minutes at 400 degrees F (200 degrees C), then reduce temperature to 350 degrees F (180 degrees C), and continue to bake until a toothpick inserted in the center of a muffin comes out clean and the edges of the muffins have browned, 10 to 14 minutes.
