This recipe is adapted from Deb Perelman's The Smitten Kitchen Cookbook.
Use perfectly ripe plums when you make this, as they have a way of being honest in this cake -- if they're too firm, you'll notice later. And at the risk of making enemies with Alice Medrich, I will tell you that I've made this without sifting, and no one complained. Sift if you wish, you won't hear from me if you don't. —Kenzi Wilbur
Preheat your oven to 350° F, and butter and flour a 9-inch springform.
In a large mixing bowl, combine and whisk the ricotta, oil, sugar, and zest together. Add one egg, whisk well; add the next, whisk again.
Sift all of the dry ingredients -- save for the confectioner's sugar -- directly over the wet ingredients you just whisked together. Mix with a spoon gently until just combined. (Use more of a folding motion toward the end -- this will help keep the gluten from developing in your batter, which will keep your cake from toughening or becoming too dense.)
Pour the batter into the cake pan, spreading it out evenly as needed. If your plums are very tart, toss them with a tablespoon of sugar or honey, and place them in the top of the cake, cut-side down. Bake for 35 minutes, or until the top is golden brown, the edges are pulling away from the pan, and a cake tester or toothpick comes out of the cake cleanly. (Depending on your oven, this could take up to 45 minutes.) Cool in the pan for 10-15 minutes, then turn out to finish cooling on a rack.
Dust with confectioner's sugar, and serve slightly warm or at room temperature. This cake wouldn't be opposed to whipped cream or whipped mascarpone, either.