First, make the streusel by adding the flour, sugar, quick cooking oats and cinnamon together. Then, cut in the butter until a you get a crumbly texture. Once mixed, set in the refrigerator while you are mixing the muffin dough (no longer than 10 minutes)
In a large bowl, combine the flour, sugar, baking powder, baking soda and salt: add berries and toss gently.
In a separate bowl, lightly beat eggs and add sour cream, oil and vanilla; mix well.
Stir into dry ingredients and just mix until combined. Do not over mix!
Fill muffin liners or greased muffin tin ⅔ full of batter and add streusel mixture to the top of each muffin.
Bake at 400 degrees for 18-22 minutes or until a toothpick comes out clean.
Cool for 5 minutes before moving to a cooling rack.
Recipe by Glitter and Goulash at http://glitterandgoulash.com/streusel-topped-blueberry-muffin-recipe/