Put the cream and yogurt in a bowl and whip together until just firm.
Put the blueberries and raspberries in a small saucepan. Cook over a low heat for 5–6 minutes until the juices start to ooze. Process with a hand blender or push through a sieve.
Stir the cream mixture into the fruit and spoon into 2 individual ramekins or glasses. Sprinkle over the dark brown sugar and chill in the refrigerator for 15 minutes until the sugar has dissolved on the top. Serve.