Position a rack in the lower third of the oven and preheat to 350°F. Coat a 9″ round cake pan (I use a spring form pan) with cooking spray. Line the bottom with parchment paper and spray the paper.
In a bowl whisk the flour, baking powder and salt. Set aside. Beat together the sugar and butter until pale and fluffy. Add eggs one at a time and beat well after each. Add the flour mixture and mix well. Scrape the batter into the prepared pan and spread evenly. Poke the plum slices into the batter, placing them close together. In small bowl, combine the cinnamon and the sugar and sprinkle over the surface.
Bake until the top is golden, the edges pull away from the pan, and the skewer or cake tester inserted into the center comes out clean, 50 to 60 minutes. Transfer the pan to a wire rack and let cool for about 30 minutes before serving.