Pre heat over to 180 C
Sift the flour, baking powder and salt in a bowl.
Using an electric mixer, cream the butter butter and 1 cup of sugar together until light and fluffy. Add the eggs, one at a time making sure each is well incorporated before adding the next. Add the dry ingredients and mix briefly unti just combined.
Spoon the cake batter into a greased 23 cm / 9-inch springform cake tin with the bottom lined with baking paper and smooth the top. Arrange the plums, skin side up evenly over the batter. Sprinkle the top with the lemon juice, then the cinnamon and remaining two tablespoons of sugar
Bake until the cake is golden and a sharp knife inserted into a center part of the cake comes out clean, about 45 to 50 minutes. If your cake is over browning, loosely cover with foil from about ¾ of the way through the baking time. Cool on rack.
Once cool, leave at room temperature over night before taking it out of the tin. You could serve this with whipped cream dolloped on top or with a light sifting of icing sugar.