Preheat your oven to 350°F (175°C) and prepare a loaf pan.
In a large bowl, mash the ripe bananas until smooth.
Add the chopped plums, both sugars, and vegetable oil into the bowl with the mashed bananas and mix until combined.
Beat in the eggs and vanilla extract until blended.
Sift together the flour, cocoa powder, baking powder, baking soda, and salt in a separate bowl.
Gradually fold the dry ingredients into the wet mixture until just combined.
Pour the batter into the prepared loaf pan and smooth the top.
Bake in the preheated oven for 55-65 minutes, or until a toothpick inserted comes out clean.
Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
