Fold a grid pattern on parchment paper to fit the bottom of the baking pan, then place it inside. Trim any excess paper.

Slice half of the banana and mash the other half. Mix the mashed banana, egg, and milk together. Gradually add the dry ingredients and mix well.

Pour half of the mixture into the baking pan, evenly place chocolate pieces and banana slices on top, then pour the remaining mixture over it. Place banana slices on top. Tap the pan gently to release air bubbles and level the surface.

Preheat the oven. My small oven has four heat settings and a maximum of 15 minutes per setting. I baked it at 250°C (about 480°F) for a total of 40 minutes. Halfway through, I placed a sheet of aluminum foil on top to prevent over-browning. Insert a chopstick into the center; if it comes out clean, the cake is done.