Double chocolate muffins

100g butter, melted
1 c sugar
2 eggs
1 c yoghurt
1/2 c milk
1/2 tsp vanilla essence
2 c flour
1 1/2 tsp baking powder
1/4 c cocoa
1/4 c chocolate chips

Mix butter and sugar in a bowl. Beat in eggs, yoghurt, and vanilla essence. Add to this the sifted cocoa, flour and baking powder, then mix.
Lastly add chocolate chips. Place in muffin tins. Bake 180°C 20 mins, or top bounces back when pressed.