For the chocolate muffins, preheat the kitchen oven to 180 degrees. Line muffin tins with appropriate paper cups.
Sift the flour, baking powder and cocoa into a large enough bowl and stir in the sugar. Press a well in the center. In a separate baking bowl, stir through eggs with sour cream, milk and oil, then add to dry ingredients in the bulge. Stir well and gradually work in all the flour mixture from the sides to make a thick, creamy batter.
Stir the white and dark chocolate chips into the batter.
Using a spoon, fill the muffin cups almost to the top with the chocolate mixture.
Bake for 25-30 minutes, until the chocolate muffins are well risen and firm to the touch. Let cool on a cooling rack, then dust lightly with cocoa powder.