Boil or steam sweet potato until tender.
Preheat oven to 180 C (355 F) and line a baking tray with baking paper.
In a large bowl, mash banana, sweet potato and softened butter together until well mashed and combined.
Whisk the egg in a small bowl or cup to break up the yolk, then add it to the mixing bowl and mix it through.
Add flour and mix until well combined.
Drop spoonfuls of the mixture onto the prepared baking tray, evenly spaced.
Bake for 14-15 minutes, until firm.
Makes about 20.
Store in the fridge for up to 3 days, or freeze in a freezer-safe container.