Make the muffins: Preheat oven to 425F and lightly grease a 12-cup muffin tin. Set aside.
In a large bowl, whisk mashed banana, butter, yogurt, sugar, eggs, milk, and vanilla until smooth. Add flour, baking powder, salt, and coconut to bowl and gently stir until batter is just combined.
Fill muffin cups 3/4 full with batter and smooth tops. Set aside for a moment.
Make the streusel: Wipe bowl clean and combine all streusel ingredients until crumbly. Sprinkle over muffin batter, gently pressing to adhere topping.
Bake muffins at 425F 5 minutes then reduce heat to 375F and continue baking 13-15 minutes until muffins test done with a toothpick. Allow muffins to cool in pan 10 minutes, then transfer to a wire cooling rack. Enjoy!