Sift the flour, baking soda and cocoa into a large mixing bowl.
Melt the butter and add the castor sugar, egg, yoghurt, milk, vanilla and 1/4 cup of chocolate chips.
Mix until you have a relatively smooth consistency.
Fold the dry ingredients (flour, baking soda, cocoa) into the combined liquid ingredients (butter, castor sugar, egg, yoghurt, milk, vanilla and chocolate chips) until the dry ingredients are dampened but not smooth.
Spoon the mixture into 12 medium sized muffin pans that have been well coated with non-stick cooking spray.
Sprinkle with an additional 1/4 cup of chocolate chips.
Bake at 200 degrees celcuis for 10-15 minutes.
Leave to stand in the muffin pans for about 3 minutes before putting on wiring rack to cool.