Heat oven to 180°C. Coat a 9-inch springform [I have this one] with butter or nonstick spray. For even easier removal, line the bottom with a round of parchment paper.
In a large bowl, beat butter and sugar together with an electric mixer until fluffy and lighter in color. Add the eggs, one at a time and scraping down the bowl. Sift flour, baking powder, and salt over batter and mix it until just combined.
Spoon batter into prepared cake pan and smooth the top. Arrange the plum tightly in the pan, all over the batter, covering it. Sprinkle the top with lemon juice, then with a sugar-cinnamon mix.
Bake until cake is golden and a toothpick inserted into a center part of the cake comes out free of batter (but of course not plum juice), about 45 to 50 minutes. Cool on rack.
