Preheat the oven to 350°F (180°C). Grease with butter a 8 inch (20cm) pan and line with parchment paper.
In a medium bowl, stir together flour, cocoa powder, baking powder, baking soda and salt.
In another bowl mash banana with a fork and set aside.
In a large bowl add egg, both sugars, vanilla extract and vegetable oil. Mix until well combined. Add mashed banana and mix to combine.
With the mixer on low, incorporate the flour mixture.
Pour batter into the prepared pan.
Bake for 10-12 minutes, until a toothpick inserted into the cake comes out clean. Let cool completely and transfer to a serving plate.