How to Make It
Preheat oven to 425°F. Line a 12-cup muffin pan with 12 paper cupcake liners. Whisk together baking powder, salt, 1 1/3 cups of the flour, and 1/3 cup of the cornmeal in a medium bowl; set aside.
Stir together 1/4 cup of the sugar, 3 tablespoons of the melted butter, and remaining 1/3 cup flour and 2 tablespoons cornmeal in a separate bowl until crumbly; reserve for streusel topping.
Whisk together buttermilk, egg and yolk, vanilla, and remaining 3/4 cup sugar in a medium bowl until well combined. Make a well in center of flour mixture. Slowly pour buttermilk mixture into flour mixture, stirring with a spatula until just combined (it will be lumpy). Fold in berries and remaining 5 tablespoons melted butter.
Scoop batter into prepared pan until each cup is three-fourths full (about 3 tablespoons each). Top each with about 1 tablespoon reserved streusel topping. Bake in preheated oven until a wooden pick inserted in center of each muffin comes out clean, about 18 minutes. Remove from oven; cool in pan 5 minutes. Cool muffins on a wire rack, about 10 minutes.