Preheat the oven to 180°C (350°F) and grease a 20cm (8 inch) square baking tin.
Add the banana to a large bowl and mash with a fork. Whisk in the oat milk, olive oil, and vanilla until combined. whisk together the flour, coconut sugar, cocoa, baking powder, baking soda, salt, and vanilla. Stir in the chopped chocolate.
Add the the flour, coconut sugar, cocoa, baking powder, baking soda, and salt. Mix until just combined, but be careful not to over mix.
Add the chocolate chips and fold to evenly incorporate.
Transfer the batter into the prepared cake tin. Bake for 40-45 minutes, or until a wooden skewer inserted into the middle of the cake comes out clean.
Remove from the oven and cool for ten minutes in the tin before carefully removing and cooling completely on a wire rack.
If topping with ganache, the cake should be refrigerated. Otherwise, it can be stored in an airtight container in a cool place for 2-3 days. This cake freezes well.