Preheat the oven to 350 F.
Add the banana to a 7-8 inch square baking dish and mash with a fork until it resembles a paste.
Add the milk and vanilla and stir to combine.
Add everything else, except for the berries or other add-ins and stir until fully combined.
Mix in your choice of add-ins such as fresh or frozen berries, chocolate chips, nuts, seeds, grated carrot or chopped dried fruit. If you're using frozen berries, it's suggested to sprinkle them over top of the bars and press them in a bit rather than mix them in.
Bake for 30 minutes until golden brown and firm.
Remove from the oven and let sit cool in the pan for 20-30 minutes before slicing. Cooling is not required but the bars will be firm and set if you let them cool fist. Serve warm with your favourite oatmeal toppings like peanut butter and yogurt or enjoy as a snack bar.