For the topping: In a medium bowl, stir together the 1/2 cup flour, 1 teaspoon cinnamon and 1/2 cup brown sugar. Add the 4 tablespoons butter and combine with a fork until the topping resembles coarse crumbs. Set aside.
Preheat over to 350 degrees.
Place paper baking liners in muffin pans. In a separate medium bowl, sift together flour, baking powder, baking soda and salt.
With electric mixer fitted with paddle attachment, cream the butter and sugar until light and fluffly, about 5 minutes.
At low speed, beat one egg at a time, then add vanilla, sour cream and milk until combined. Add 1/3 of the dry ingredients at a time and beat until just combined. Carefully fold in blueberries into batter.
Scoop batter into muffin pans filling each cup to the top. Sprinkle topping evenly over batter. Be generous.
Bake 30-35 minutes until cake tester comes out clean.
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Blueberry Muffins With Streusel Topping Recipe (cont.)