Preheat the oven to 180C
Add the peanut butter, banana, and sugar to a bowl and mix to combine. Add the egg, mix again, then stir in the oats.
Transfer the mixture to a lined backing dish, or scoop onto a tray into whatever shaped cookies you like. They won't change shape during cookies, so if you want a flat cookie, press them into that shape before baking.
Bake for 40 minutes for the whole slice, or 20-30 if making cookies. The cooking time will vary quite a lot based on how thick it is, so it's best to keep an eye on it from the 20 minute mark and pull it out when it's golden and crisp on the outside. They will also firm up once they cool, so don’t worry if they are a little soft straight from the oven.
Optionally, drizzle with melted chocolate to finish.