Preheat the oven to 350°F (175°C).
Grease a loaf pan and line it with parchment paper.
In a large bowl, combine mashed bananas, melted butter, and sugar.
Add in beaten eggs and vanilla extract, mixing until well combined.
In a separate bowl, mix the flour, cocoa powder, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Fold in the chopped plums gently using a spatula.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 50-60 minutes, or until a toothpick inserted comes out clean.
Let the cake cool in the pan for 10 minutes before transferring to a wire rack.
Slice and serve once cooled completely.