Melt the chocolate and butter in a double boiler and allow to cool slightly. Beat eggs and sugar until frothy (approx. 10 minutes), then add the yogurt, rum and melted chocolate. Sift in the flour and baking powder and stir until smooth.
Spread the dough on a baking sheet lined with baking paper and bake in a preheated oven at 180 degrees for 45 minutes. Make a chopstick test.