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Mix the dry ingredients in a bowl and set aside.
Cream the butter, sugar, egg, and vanilla together to make a thick batter.
Add in the dry ingredients and combine.
Add in the milk last. Your batter will be rich and pale yellow.
Pour into a greased 9-inch springform pan.
Slice the plums and decorate the top with them, then sprinkle the remaining sugar on top.
Bake in a preheated 350°F oven for 60-70 minutes.
Let rest in the pan for 30 minutes, then run a knife around the sides and unmold.