Preheat oven to 400 and spray 12 muffin cups with non-stick spray.
Place the banana in a small mixing bowl and mash with a fork. You should have 1/2 cup mashed banana. Stir in the yogurt, egg and vanilla just until combined. Set aside.
Place the muffin mix in a large mixing bowl and make a well in the center. Pour the yogurt mixture into the well and stir just until combined.
Pour the blueberries that come with the muffin mix into a strainer, rinse under cold water and drain well. Fold them into the batter until just combined. The batter will still be a little bit lumpy.
Spoon 1/3 cup batter into each prepared muffin cup, filling it 3/4 full. Set aside.
To make the topping, place the pecans, brown sugar and flour in a small bowl and stir together. Add the butter, mashing it with a fork until the mixture is crumbly.
With your hands, sprinkle a heaping teaspoon of the topping on each muffin. Place the pan in the oven.
Bake the muffins until the pecan topping is deep golden and the muffins just spring back when lightly pressed with your finger, about 20 to 23 minutes.
Cool on a wire rack for 5 minutes before removing from pans. Cool 15 minutes more before serving.