Preheat your oven to 350°F (175°C). Line a muffin tin with liners or spray it with cooking spray to prevent sticking.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, salt, nutmeg, and cinnamon. Set aside.
In a medium mixing bowl, combine the melted butter, light brown sugar, and granulated sugar. Mix until well combined. Then add the eggs, vanilla extract, sour cream, and mashed ripe banana. Whisk until the mixture is smooth and creamy.
Pour the wet ingredients into the dry ingredients. Gently fold the mixture with a rubber spatula until just combined. Be careful not to overmix; a few lumps are perfectly fine.
Gently fold in the blueberries and toasted walnuts, ensuring they are evenly distributed throughout the batter.
Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full. If desired, sprinkle the tops with sparkling sugar for a delightful crunch.
Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean. The tops should be golden brown and spring back when lightly pressed.
Remove the muffins from the oven and allow them to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely. Enjoy warm or at room temperature!
