Break the chocolate into squares and put them in a large heatproof bowl over a saucepan of simmering water. Leave until melted, then remove from the heat and allow to cool a little. Whisk in the egg yolks and fold in the flour.
Whisk the egg whites and caster sugar in a medium bowl until they form soft peaks. Beat a spoonful of the egg whites into the chocolate mixture to loosen it before gently folding in the rest.
Put the raspberries into 4 lightly greased ramekins, pour over the chocolate mixture and cook in a preheated oven, 190°C (375°F), Gas Mark 5, for 12–15 minutes until the soufflés have risen.
Sprinkle the soufflés with icing sugar and serve with extra raspberries, if liked.