Preheat your oven to 350°F. Butter a 9-inch springform pan or round cake pan, then line the bottom with parchment paper.
In a small bowl, whisk together flour, baking powder, and salt. Set aside.
In a large bowl, beat butter and sugar with an electric mixer on medium-high speed for about 4 minutes, until light and fluffy. The mixture should look pale yellow and slightly increased in volume.
Beat in eggs one at a time, mixing well after each addition. Add vanilla and mix briefly to combine.
Reduce mixer speed to low and gradually add the flour mixture, alternating with milk — begin and end with flour. Mix until just combined; do not overmix. The batter will be thick but pourable.
Arrange plum halves cut-side up on top of the batter in a single layer (you can press them down slightly). Sprinkle 2 tbsp sugar evenly over the plums and cake surface.
Bake for 35–40 minutes, until the top is golden brown and a toothpick inserted into the center comes out clean or with just moist crumbs. If the top browns too quickly, tent loosely with foil halfway through baking.
Cool in the pan on a wire rack for 10 minutes before removing the ring. Serve warm or at room temperature.
