Preheat the oven to 110C/90C Fan/Gas ½. Line a large baking tray with silicone or baking paper.
Whisk the egg whites in a clean bowl using an electric whisk, or in the bowl of a freestanding mixer, until soft peaks form when the whisk is removed.
Add 300g/10½oz of the caster sugar and continue to whisk until the mixture is smooth and glossy and stiff peaks form when the whisk is removed. You should hear the machine dropping down a gear as the meringue mixture reaches the correct consistency. Reduce the speed of the whisks and add the cornflour and vinegar. Whisk until very stiff.
Transfer the meringue mixture to a piping bag. Pipe an outline of a Christmas tree shape onto the prepared baking tray. Pipe a rectangular trunk at the bottom. Fill the outline with the remaining meringue mixture, then smooth the top using a palette knife. With the last of the meringue mixture, pipe around the edge of the Christmas tree shape to form a raised edge. Pipe little balls on each of the points to resemble baubles.
Bake the Christmas tree in the oven for 1½-2 hours, or until firm, then remove from the oven and set aside to cool.
Meanwhile, make the filling. Bring the remaining caster sugar and 150ml/5fl oz water to the boil in a saucepan. Stir in the ground cloves, ground ginger, cinnamon stick, and lemon and orange peel and boil for a further 4-5 minutes, or until thickened and syrupy. Stir in the blackberries, then remove the pan from the heat and set aside until completely cool.
When the meringue and fruit syrup have cooled, whisk the cream and vanilla bean paste in a bowl until soft peaks form when the whisk is removed, then stir in the crème de cassis.
To assemble, transfer the meringue to a serving platter and spread the cream on top. Spoon over the fruit syrup. Serve immediately.