How to Make It
Preheat the oven to 400°. In a medium saucepan, combine the raspberries and blackberries with the granulated sugar, water, vanilla and lemon juice. Bring to a simmer over moderate heat, stirring often. Stir in the cornstarch mixture and cook, stirring, until thickened, about 1 minute.
In a large bowl, beat the egg whites until they hold very soft peaks. Gradually beat in the superfine sugar, 2 tablespoons at a time, and continue beating until glossy peaks form, about 3 minutes longer. Gently swirl in 3/4 cup of the berry mixture, leaving streaks in the beaten whites.
Spoon the remaining berry mixture into six 1/2-cup ramekins. Top with the soufflé mixture, mounding it slightly. Sprinkle with the pistachios. Bake the soufflés for 6 minutes, or until golden on top. Serve.