In a large bowl whisk together egg, mashed bananas, vanilla extract, salt, maple syrup, peanut butter, and milk until light and fluffy.
Add in flour, rolled oats, and baking powder; stir until well combined. Set aside 2/3 of the batter.
Add the cocoa powder to the remaining batter and mix until combined.
Spoon batters into the prepared pan in 2 layers, alternating spoonfuls of vanilla and chocolate to simulate a checkerboard. To create marbling, run a table knife (or wooden skewer) through the batters in a swirling motion.
Bake until a cake tester comes out clean, 35-45 minutes. Transfer pan to a rack to cool 10 minutes. Turn out cake on to a serving plate, frost with confectioner sugar and serve.