Preheat the oven to 350°F (175°C) with a rack in the center. Grease a 9-inch springform pan and line the bottom with parchment.
In a large bowl, beat the unsalted butter and the 1 cup granulated sugar with a hand mixer or stand mixer on medium speed until pale and fluffy, 3–4 minutes.
Add the large eggs one at a time, beating well after each addition and scraping the bowl.
In a separate bowl, whisk together the all-purpose flour, baking powder, and fine salt. Add to the wet mixture and mix on low just until no dry spots remain; the batter will be thick.
Scrape the batter into the prepared pan and spread it evenly to the edges.
Arrange the italian prune plums, skin side up, in snug concentric circles on top of the batter, pressing them lightly into the surface. Drizzle the lemon juice evenly over the fruit.
In a small bowl, stir together the 2 tbsp granulated sugar and the ground cinnamon, then sprinkle the mixture evenly over the top.
Bake until the top is golden, the plums have sunk slightly and release syrupy juices, and a toothpick inserted into the cake (not a plum) comes out with a few moist crumbs, 45–55 minutes.
Cool on a rack for 15 minutes, then loosen and remove the springform ring. Cool at least 30 minutes more before slicing. Serve warm or at room temperature.