Preheat oven to 400°F (204°C). Line a 12-cup muffin tin with paper liners and set aside.

In a large bowl, sift together 2 3/4 cups (330 g) all-purpose flour, 1 Tablespoon (14 g) baking powder, 1/2 teaspoon salt, 1/4 teaspoon ground nutmeg, and 1/4 teaspoon ground cinnamon. Set the dry mixture aside.

In a separate large bowl, mash 1 cup (227 g) ripe banana until mostly smooth.

Add to the mashed banana: 1 cup (227 g) unsalted butter (melted and slightly cooled), 1/2 cup (106 g) light brown sugar (packed), 1/2 cup (99 g) granulated sugar, 3 large eggs (room temperature), 2 teaspoons pure vanilla extract, and 3/4 cup (170 g) full-fat sour cream. Whisk until the wet ingredients are fully combined and smooth.

Pour the dry ingredients into the wet ingredients. Using a rubber spatula, fold and mix gently until just combined—do not overmix.

Fold in 1 pint (292 g) fresh blueberries and 1/2 cup toasted walnuts (finely chopped) until evenly distributed, mixing only until combined.

Divide the batter evenly among the prepared muffin cups, filling each cup all the way to the top.

Press a few extra blueberries onto the top of each muffin. If using, sprinkle each muffin with the 2 Tablespoons sparkling sugar.

Bake on the center rack for 24 to 26 minutes, or until the tops are puffed and golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs attached.

Remove the muffins from the oven and let them cool in the pan for at least 15 minutes before serving warm or transferring to a cooling rack.