Blueberry Cinnamon Muffins

¼ cup regular oats
¼ cup flour
½ cup brown sugar
1 tsp. ground cinnamon
1 tbsp. softened butter
¼ cup butter, softened
1 cup granulated sugar
2 eggs, lightly beaten
1 tsp. vanilla extract
2 cups all-purpose flour
1 tsp. baking soda
½ tsp. baking powder
½ tsp. Salt
1 cup milk mixed with ¼ cup sour cream (or 1 ¼ cups buttermilk)
1 cup fresh blueberries, dusted with flour

Preheat oven to 350º F, and spray muffin pans with non-stick cooking spray. Mix first five ingredients in a large bowl and reserve for streusel topping. Beat butter and sugar until fluffy, and add eggs until blended. Stir in vanilla. Combine flour, soda, baking powder, and salt. Add flour mixture to butter mixture, alternating with sour milk (and ending with flour). Fold in blueberries. Pour batter evenly into muffin cups and top with streusel mixture. Bake for 15-20 minutes until golden. Cool for five minutes, and remove from muffin pans. Makes 15 muffins.