Preheat your oven to 350°F (175°C) and line a mini muffin tin with paper liners or grease it lightly.
In a bowl, combine the mashed bananas, chopped strawberries, milk, egg, vanilla, and melted butter or coconut oil. Mix until well combined.
Add the whole wheat flour and baking powder to the wet mixture. Stir until just combined (don't over mix!) The batter should be thick but slightly spoonable.
Spoon the batter into the mini muffin tin, filling each cup to the top.
Bake for 13-15 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for a few minutes before transferring them to a wire rack to cool completely.
Serve and enjoy!