Preheat oven to 375°F with a rack in the middle of the oven.

In a large bowl, whisk melted butter and brown sugar and 1/2 cup granulated sugar until well combined, about 1 full minute. Add eggs and vanilla, whisking until combined. Then add mashed banana and sour cream, whisking until combined.

In a separate bowl, combine the flour, baking soda, baking powder, cinnamon, nutmeg (optional) and salt. Add dry ingredients to the wet ingredients and use a rubber spatula to lightly stir the batter until almost fully combined. Toss 1 cup blueberries with 1 tablespoon of flour, then stir into the batter.

Grease 12 standard-sized muffin cups or line with paper liners. Divide the batter evenly among the cups, filling almost all the way to the top. Using the remaining 1/4 cup blueberries, add a few blueberries to the top of each muffin. Sprinkle the tops with granulated sugar. Bake for 18-22 minutes, until a toothpick inserted into the center comes out clean. Place the pan on a wire rack to cool for 10 minutes, then remove the muffins from the pan and place them directly on the rack to cool..