Preheat oven to 160°C. Grease six 10cm round, loose-based flan tins; place on oven tray.
Melt butter in a medium saucepan. Add sifted cocoa and whisk over low heat until mixture boils. Remove from heat; whisk in jam and ½ cup of the sugar. Stir in eggs, then sifted flour and soda. Divide mixture among tins.
Beat cream cheese, remaining sugar and egg yolk in a small bowl with electric mixer until smooth. Stir in raspberries.
Drop spoonfuls of cream cheese mixture over chocolate mixture; pull a knife backwards and forwards several times for a marbled effect. Bake about 30 minutes. Serve tarts warm or cold with whipped cream, if you like.