Prepare a baking dish by greasing the inside with butter and dust with flour.
Sift the flour, cocoa and salt together in a large mixing bowl.
Whisk the sugar, melted butter and coconut oil together with the eggs, then whisk in the milk.
Slowly add this egg mixture to the flour using a wooden spoon until you have a thick chocolate batter.
Pour the batter into the prepared baking dish, then evenly place the plums into the batter, top with the coconut sugar and boiling water and bake for 40 minutes or until cooked.