1. Preheat the oven to 425 F. Line a muffin tin with six muffin liners. Note: I have a 6-cup muffin pan, but if you’re working with a 12-cup muffin pan, leave one empty well between each liner.
2. In a mixing bowl, whisk the melted butter, granulated sugar, and light brown sugar together until combined. Whisk in egg and vanilla extract, then buttermilk and sour cream.
3. Whisk the dry ingredients (flour, baking soda, baking powder, salt, and ground cinnamon) in a large mixing bowl until totally combined.
4. Place the fresh blueberries into a separate, smaller bowl. Sprinkle one tablespoon of the dry ingredients (flour mixture) over the blueberries, then toss to coat them in the flour mixture. Set aside.
5. Pour the wet ingredients into the bowl of dry ingredients, then use a spatula to mix them together until just combined. (Avoid over-mixing; a few small clumps of flour are ok!)
6. Toss the coated blueberries. Fold the blueberries into the batter until just combined, but evenly dispersed.
7. Lightly spray the muffin liners with baking spray, then divide the batter between them, filling each liner to the top. Set them aside while you make the topping.
8. Mix the topping ingredients together in a small dish. Top each muffin with 1-2 TBSP.
9. Bake the muffins at 425 F for five minutes. While keeping the oven door closed, turn the oven temperature down to 350 F. Bake for another 10-12 minutes until the tops are golden brown, and a clean toothpick inserted into the center of a muffin comes out clean.
