Preheat your oven to 350°F (175°C).
Grease and flour a 9-inch loaf pan.
In a large bowl, cream the softened butter and sugar together until light and fluffy.
Add the mashed bananas, eggs, and vanilla extract; mix well.
In a separate bowl, combine flour, cocoa powder, baking soda, and salt.
Gradually add the dry mixture to the banana mixture, mixing until just combined.
Fold in the chopped plums and chocolate chips.
Pour the batter into the prepared loaf pan.
Bake in the preheated oven for 55-65 minutes, or until a toothpick comes out clean.
Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack.
Slice and serve your delicious banana chocolate plumcake.