- Preheat the oven to 160°C / 325°F / gas 3.
- Draw a 20 cm circle on a sheet of non-stick baking paper, turn over and place on a baking sheet.
- Spread out the brown sugar on another baking sheet and dry in the oven for 8–10 minutes.
- Sieve to remove any lumps.
- Whisk the egg whites in a bowl until stiff.
- Add half the dried brown sugar, l5 ml / 1 tbsp at a time, whisking well after each addition.
- Add the vinegar and vanilla extract, then fold in the remaining sugar.
- Spoon the meringue on to the drawn circle on the paper, making a hollow in the center.
- Bake for 45 minutes, then turn off the oven and leave the meringue in the oven with the door slightly open, until cold.
- Place the blackberries in a bowl, sprinkle over the creme de cassis and leave to macerate for 30 minutes.
- When the meringue is cold, carefully peel off the non-stick baking paper and transfer the meringue to a serving plate.
- Lightly whip the cream with the icing sugar and spoon into the center.
- Top with the blackberries and decorate with small blackberry leaves, if liked.
- Serve at once.