Preheat the oven to 350°F (175°C).
Mash the ripe bananas in a large mixing bowl.
Add the brown sugar and mix until well combined.
Incorporate the eggs and mix in the oil and vanilla.
Sift together flour, cocoa powder, baking powder, and baking soda in a separate bowl.
Gradually add the dry ingredients to the wet mixture.
Fold in the chopped plums and dark chocolate chips.
Pour the batter into a greased loaf pan.
Bake for 50-60 minutes or until a toothpick comes out clean.
Let the cake cool in the pan for 10 minutes before transferring to a wire rack.
Slice and enjoy your delicious Banana Chocolate Plumcake.