Preheat the oven at 180° C. Sieve together refined flour, baking powder, baking soda and cocoa powder into a bowl.
Place dark chocolate and butter in another bowl and melt in a microwave oven for a minute. Whisk well till well blended. Add castor sugar to the chocolate mixture and mix well. Fold in the flour mixture.
Add yogurt and milk and mix till the mixture is smooth without any lumps. Pour the mixture into four individual silicon muffin moulds and bake in the preheated oven for seven minutes.
Take the moulds out the oven and let them cool slightly.Remove from the moulds and serve hot.