Preheat oven to 400 degrees (F). Line a 12 cup muffin tin with paper liners and it set aside.
In a large bowl, sift together the flour, baking powder, nutmeg, cinnamon, and salt; set aside.
In a separate large bowl, whisk together the melted butter, both sugars, eggs, vanilla extract, sour cream, and mashed bananas, mixing until ingredients are fully incorporated.
Add the dry mixture of ingredients to the wet mixture of ingredients and, using a rubber spatula, mix until just combined. Do not over mix or your muffins will be dry!
Fold in the blueberries and walnuts, mixing just until combined.
Divide the batter evenly among prepared muffin tins, filling each tin all the way to the top.
Press a few extra blueberries into the top of each muffin and then add sprinkle with sparkling sugar, if using.
Bake, in the center of the middle rack, for 24 to 26 minutes, or until the tops are puffed up and richly golden brown, and a toothpick inserted in the middle comes out clean, or with a just a few moist crumbs attached.
Cool muffins at least 15 minutes in the pan before serving warm or transferring to a cooling rack.
NOTES
If your oven runs hot, bake at 375, for 25 to 26 minutes, or until golden brown and bake though.
Bake muffins on a rack, positioned in the middle of your oven.
If you frequently have burnt muffin bottoms, I suggest placing your muffin tin on baking sheet before baking. The baking sheet will help prevent this!
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