Use the butter to grease 4 x 200 ml (7 fl oz) ramekins and then coat evenly with a little of the caster sugar, tipping out the excess sugar. Set the ramekins on a baking sheet.
Purée the blackberries and raspberries in a food processor or blender, reserving a few of the berries to decorate, then pour the purée into a bowl. Alternatively, the berries can be rubbed through a fine sieve to make a smooth purée.
Whisk the egg whites until stiff but not dry in a large, perfectly clean bowl. Gradually sprinkle in the remaining caster sugar, whisking continuously, and continue whisking until the whites are stiff and shiny.
Gently fold the egg whites into the berry purée, then spoon the mixture into the prepared ramekins. Bake immediately in a preheated oven, 190°C (375°F), Gas Mark 5, for 15 minutes or until risen and golden.
Dust the soufflés with icing sugar and decorate with the reserved berries. Serve immediately, with custard or ice cream, if liked.