Preheat oven to 180° C.
Sieve together refined flour, baking powder, soda bicarbonate and cocoa powder into a bowl.
Place dark chocolate and butter in another bowl and melt in a microwave for a minute. Whisk 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 individual silicon muffin moulds, place the moulds on a baking tray. Place the tray in the preheated oven and bake for eight to ten minutes.
Remove the cakes from the moulds and let them cool slightly.
Arrange the cakes on serving plates and serve warm.