Preheat the oven to 400F. Put the plums and blackberries into a bowl and sprinkle with the sugar, vanilla and cinnamon. Set aside.
In a medium bowl, whisk together the icing sugar, ground almonds, flour and salt. Whisk the eggs until they’re frothy; add to the icing sugar mixture along with the melted butter.
Pour into a buttered or otherwise greased pie plate or 8 or 9-inch baking dish, and top with the fruit, scattering any excess sugar (or pouring juices) overtop. Bake for 45-50 minutes, covering with foil for the final 10 minutes, until the cake is deep golden and the fruit is bubbling. Serves 4-6.