Blueberry Yogurt Streusel Muffins

1/4 cup butter, softened
1/4 cup mashed banana
1 Lg. egg
1 cup plain yogurt
3/4 cup sugar
1 tsp. vanilla extract
2 cups all-purpose flour
1 tsp. Baking powder
1 tsp. Baking soda
1 cup fresh or frozen blueberries
Dry yellow cake mix **see note**

Cream sugar and butter, add banana and egg, stir in yogurt and vanilla ,beat well.
Add flour, baking powder and soda, stir just till all combined. Gently fold in blueberries.

Spoon into 18 greased muffin tins, and sprinkle each muffin with a scant teaspoon of dry cake mix.
Bake at 400* for 12-15 minutes. Let cool five minutes, remove from muffin pan and finish cooling on a wire rack.

**NOTE: You may have noticed I use yellow cake mix in small quantities in several of my recipes, since I don't use a whole package at a time. I keep a  lidded container in the fridge with it all the time and just measure out what I need at a time. **