Preheat oven to 170°C. Line a baking tray with parchment paper & set aside.
In a small bowl, combine the flour with the bicarbonate of soda; whisk.
In a medium bowl beat together the butter and sugars with an electric whisk until light & fluffy. Once combined, beat in the vanilla & the egg, mixing well. Add in the mashed banana, mixing until well combined and homogenous.
Using a rubber spatula, gradually incorporate the dry mixture to the wet - being sure not to overmix. Add the oats and the chocolate chips, and stir to combine.
Divide the dough into 18 equal size pieces, roughly a 45g each. Use the palm of your hand to flatten each ball slightly, and place on the prepared tray.
Bake in the preheated oven for 12-15 minutes, or until lightly golden and crispy on the edges. Leave to cool on the tray for 5-minutes, before moving to a wire rack to cool completely.