Preheat oven to 220 C (430 F) and brush a baking tray with butter or olive oil.
Allow puff pastry sheet to thaw a little so it is flexible.
Cut pastry into 9 equal sized squares by cutting it in thirds one way, turning it 90 degrees, and cutting it in thirds again.
Place spoonfuls of banana in the middle of each square.
Fold the squares in half, corner to corner. Be careful not to squash the banana out the sides. If you can't keep the banana from coming out, just take out a little of the banana and try again.
Press the edges together gently with a fork.
Place on baking tray and bake for 10-12 minutes or until puffed and golden.
Allow to cool for at least 5 minutes before giving to your toddler, as although the pastry cools quickly the banana inside will be super hot.