3/4 cup unsalted butter, melted and slightly cooled
Place the blackberries and plums in a bowl with the vanilla, granulated sugar, bay leaves, and 1/2 teaspoon of the cinnamon. Set aside for 30 minutes. Don’t be tempted to leave them sitting around for longer than this, as the fruit will become too juicy.
Preheat the oven to 400°F.
Mix the flour, confectioners’ sugar, almond meal, the remaining 1/2 teaspoon of cinnamon, and salt in a separate large bowl. Set aside.
Lightly whisk the egg whites by hand for 30 seconds, so they just start to froth. Stir into the flour mixture, along with the melted butter, until combined.
Tip the batter into a 23 x 33 centimeter parchment-lined baking dish and top evenly with the fruit and juices. Bake for 60 minutes, covering the dish with foil for the final ten minutes, until the cake is golden brown and the fruit is bubbling. Set aside for ten minutes before serving.