Preheat the oven to 180ºC and line a 7-inch square tin with non-stick baking paper.
Add the bananas to a bowl and mash well with a fork. Add the egg, sweetener, and cinnamon, and mix.
Add the oats and chocolate chips, and mix until combined.
Scrape the mixture into the lined tin, and smooth out the top — until flat.
Pop the tin into the oven for 20 minutes, until the mixture is just set and the top is turning golden brown. Remove from the oven and cool in the tin. Slice into 8 bars.