Heat oven to 160 deg C fan bake. Spray a 12-hole standard muffin tin with baking spray. Place softened butter and sugar in a bowl, then beat until pale and creamy.
Beat in eggs, sour cream and zest of one orange. Sift flour and baking powder over the creamed mixture and stir to combine.
Spoon mixture into prepared muffin tin. Arrange a few slices of plum onto the surface of each little cake. Sprinkle a little raw sugar over each.
Bake for 25 to 30 minutes or until a skewer inserted in the centre of one cake comes out clean. Remove to wire rack to cool.
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