Chill the bowl and blade of a food processor in the refrigerator.
To make the cream hearts, blend the mascarpone, cream cheese and castor sugar in the food processor until smooth. Scrape the sides of the bowl with a plastic spatula occasionally to keep the mixture evenly distributed.
Add the cream and yoghurt and blend briefly to incorporate.
Line 6 porcelain heart moulds with a double layer of wet muslin and carefully spoon in the cream mixture until the moulds are full and the surface is even.
Put the moulds on a tray with a lip (to catch the whey), then cover with plastic film and refrigerate for at least 8 hours before serving.
To serve, arrange the raspberries and blackberries in the centre of each serving plate and spoon the raspberry sauce over the berries.
Lift the cream hearts out of their moulds using the muslin, then invert each heart onto the berries and carefully remove the muslin.