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.
Whisk the dry ingredients (flour, baking soda, baking powder, salt, and ground cinnamon) in a large mixing bowl until totally combined.
In a separate 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.
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.
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!)
Toss the coated blueberries (and any dry ingredients that have settled to the bottom of the blueberry bowl) into the bowl with the batter. Fold the blueberries into the batter until just combined, but evenly dispersed.
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 streusel.
To Make the Streusel Topping:
Place the cold butter, light brown sugar, flour, and ground cinnamon in a small mixing bowl. Use your hands or a fork to gently smash the ingredients together, squishing the butter into the dry ingredients until only small pebble-like pieces remain to form the streusel.
Divide the streusel over the tops of the muffins.
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. Remove the muffins from the oven, then allow them to cool in the pan for 5-10 minutes before serving.
Notes
Use full-fat sour cream for the best flavor and texture.To use frozen blueberries, thaw them first and pat them very dry before mixing into the batter.If the butter is too warm for the streusel, it can result in a mashed sugary butter spread. Pop it in the freezer for a few minutes, then continue crumbling it until it resembles small pebbles.