Preheat oven to 350°F (175°C), with a rack in the center of the oven. Butter the bottom and sides of your pan and line the bottom with greased parchment paper, then dust the insides of the pan with flour. Set aside.
Make the streusel:Place sugar, cinnamon and butter in a medium bowl. Mash all ingredients together with a fork or your hands until the sugar mixes with the butter. Then add the flour and keep pinching together the ingredients until you get a crumbly mix. Place it in the refrigerator to chill as you make the cake.
Make the batter:Beat butter and sugar in a medium bowl until smooth and fluffy, a few minutes. Crack in the eggs one at a time, whisking after each addition. Stir in vanilla and salt. Then sift in flour and baking powder and whisk just until combined.
Scrape batter into prepared pan and smooth the top. Remove stones from plums and slice each plum into 4 wedges. Cover the whole cake with plum slices, one next to the other, until no empty spots are left. Sprinkle with streusel.
Bake cake for 50-60 minutes, until the top is golden brown and fragrant (and a cake tester inserted in the center comes out clean. It can have plum residue on it, as those will be juicy, but you don’t want to see the actual beige batter on the cake tester.)
Cool cake for 30 minutes or more before removing from the pan. Dust with powdered sugar before serving. Keep covered in the refrigerator and eat within 2-3 days.
Notes
Make sure you use plums that are ripe. If they're too greenish on the inside and too firm, the cake won't taste as good.