In a second bowl, whisk melted butter, sour cream, milk, and egg YOLKS.
Slowly stir wet ingredients into dry ingredients, until just combined.
Let batter rest for 15 minutes while oven preheats to 400°F.
Meanwhile, beat egg whites to stiff peaks and toss blueberries with the extra flour.
After 15 minutes, gently fold the egg whites and blueberries into the batter until just combined.
Add muffin liners to a muffin tin and fill with batter about ⅔ full.
Top with granulated or coarse sugar if desired.
Bake for 5 minutes then turn oven down to 375°F and bake an additional 12-15 minutes.
Notes
Use granulated sugar, organic cane sugar, sucanat, or coconut sugar in these muffins.
Fresh or frozen blueberries both work. If using frozen, keep in the freezer until ready to toss in flour and stir in, to avoid bleeding into the batter.