Preheat the oven to 375 degrees F. Line a 12-cup muffin pan with paper liners.
Whisk the flour, cornmeal, sugar, baking powder and salt together in a large bowl. Whisk the egg, yolk, buttermilk, butter and vanilla together in a medium bowl until smooth.
Pour the buttermilk mixture into the flour mixture and stir just until the batter comes together but still has lumps. Stir in the berries. Divide the batter evenly among the prepared muffin cups and sprinkle the tops with sugar.
Bake until golden and a toothpick inserted in the center comes out clean, about 20 minutes. Transfer the muffins to a wire rack to cool. Serve warm or at room temperature.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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