Preheat over to 180-C / 350°F.
Sift the flour, baking powder, and salt into a bowl.
Using an electric mixer, cream the butter and 3/4 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 until just combined.
Line the bottom of a 23cm / 9-inch springform cake tin with parchment paper.
Spoon the cake batter into the cake tin and smooth the top.
Arrange plums, skin side up, evenly over the batter. Top with the lemon juice, cinnamon and two tablespoons of sugar.
Bake until the cake is golden and a toothpick comes out clean, about 45 to 50 minutes.
Note: If your cake is over-browning, loosely cover it with foil from about ¾ of the way through the baking time. Cool on rack.
Once cool, leave it at room temperature overnight 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.