Sour cream muffins

Preheat the oven to 220 ºC (425 ºF). Spray 12-15 muffin tin hollows with non-stick spray or line with paper cups.
Sift the self-raising flour, baking powder, cinnamon and salt together in a large mixing bowl.
Whisk the butter until light and creamy and add small quantities of the sugar, beating continuously.
Add small quantities of the egg while continuing to beat.
Add the sour cream and mix. Add the flour mixture and fold in. Add the lemon rind, plums and walnuts and fold in. Spoon a little of the batter into each of the muffin tin hollows and bake for 12-15 minutes or until done. The muffins are done when a testing skewer comes out clean.
Cool for five minutes before removing from the pan.
Sprinkle with a little sugar if desired and serve lukewarm or cold with butter.