Beat the egg and sugar until they turn white. Add the yoghurt and melted butter. Stir the mixture well and gradually add the flour mixed with the cocoa, baking powder and vanilla.
Break the chocolate into pieces and add them to the cake mixture, stirring gently. In muffin forms, arrange paper cups and distribute the mixture into them.
Bake in a preheated 356°F (180 °C) oven for about 30-40 minutes. Decorate optionally with icing and coconut.