Heat the oven on 200°C . Line 18 muffin tins with paper cases or brush oil on the bottoms. Melt the butter and chocolate together either in a double boiler or in the microwave on a low heat. Sift the flour, salt and baking powder into a bowl. Mix in the brown sugar. In another bowl mix the milk, egg and sour cream. Add the liquid ingredients and the chocolate mix to the dry ingredients all at once. Using a fork fold the ingredients together only until it is all wet. Spoon the mixture into muffin tins and bake for approximately 15 to 20 minutes. They are ready when a skewer or toothpick comes out clean. Place muffins on a wire rack to cool.