Lightly grease a standard 12 muffin pan or use muffin liners. Preheat oven to 400°
In a bowl mix the brown sugar and ½ cup flour. Using a pastry cutter (or two knives) cut 4 tablespoons cold butter into mixture. Set bowl aside.
In a separate bowl mix flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In a mixer beat 1 stick of softened butter. Add sugar.
Add eggs, sour cream, and vanilla and mix.
Gently fold in blueberries.
Add bowl of flour and baking powder mixture to blueberry mixture and gently stir.
Divide batter among muffin cups.
Top the batter with the flour/brown sugar/butter topping.
Bake for 20 minutes, or until at toothpick comes out clean.
Let cool for five minutes before removing from pan. Store covered for up to one week.