Preheat the oven to 375°F/190°C. Prepare an 8 or 9 inch square baking dish with baking spray or butter.
Melt the butter in a medium skillet over low heat. Add the chocolate and cocoa powder and stir until well mixed and smooth. Set aside.
Beat together the sugar and yogurt, then add the semolina, baking soda, vanilla and the chocolate mixture. Mix well.
Transfer the batter to the baking dish and bake for 25 - 30 minutes. It's done when a knife inserted into the center comes out clean.
Cut into 9 pieces and serve warm.