Preheat oven to 425°F (220°C). Line a 12-cup muffin tin with liners or grease well. If making streusel, combine flour and brown sugar and cut in cold butter until crumbly; refrigerate.
In a large bowl whisk together flour, sugar, baking powder, baking soda, salt, and cinnamon.
In a separate bowl whisk eggs, melted butter, sour cream, buttermilk, and vanilla until smooth.
Pour wet into dry and fold gently until just combined. Toss blueberries with 1 tbsp flour and fold in carefully to avoid bursting berries.
Divide batter among muffin cups, filling nearly to the top. Sprinkle streusel if using. Bake at 425°F for 5–7 minutes, then reduce oven to 350°F (175°C) and bake 14–18 more minutes, until a toothpick shows a few moist crumbs.
Cool in the pan 5–8 minutes, then transfer to a wire rack to finish cooling. Serve warm or at room temperature.
Notes
For taller domes, fill cups almost to the top and give the oven a short high-heat start.
Toss frozen blueberries in flour and bake a few minutes longer; don’t thaw.
Store muffins in an airtight container for up to 2 days at room temperature or freeze for up to 1 month.