Preheat oven to 180°C. Line a muffin tin with paper baskets or put 12 silicon muffin cases on a baking sheet.
Mash the bananas with a fork. Add the eggs, oil, milk and vanilla to the bowl. Mix until well combined.
Add the flour, baking powder, bicarb and cinnamon. Mix until well combined.
Peel and core the apple, then cut it into 1 cm pieces. Fold these pieces into the batter.
Divide the batter evenly between the muffin cups.
Bake the muffins for approx. 20 minutes, until a skewer inserted into the centre of a muffin comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer them to a wire-rack to cool completely.