Preheat & Prep: Preheat the oven to 400°F. Line a 12-cup muffin pan with paper liners.
Mix Dry Ingredients: In a large bowl, sift together the gluten free flour, baking powder, nutmeg, cinnamon, and salt. Set aside.
Mix Wet Ingredients: In a separate large bowl, whisk together the melted butter, coconut sugar, eggs, vanilla extract, and sour cream until well combined.
Combine Wet & Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, using a rubber spatula to mix until just combined.
Add Mix-ins: Gently fold in the blueberries until evenly distributed.
Fill Muffin Tin: Divide the batter evenly among the prepared muffin cups, filling each to the top.
Top & Bake: Press a few extra blueberries on top of each muffin and sprinkle with sparkling sugar, if desired. The granulated sugar on top of the muffins creates a delightful bakery-style crunch, adding a crisp, slightly caramelized texture that contrasts beautifully with the soft, moist interior. Bake in the center of the oven for 24-26 minutes, or until the tops are golden brown and a toothpick inserted in the center comes out clean or with just a few moist crumbs.
Cool & Serve: Let the muffins cool in the pan before serving warm or transferring to a wire rack to cool completely. Enjoy!