Prepare the pan and oven: Preheat the oven to 350°F (175°C). Grease a 9-inch (23-cm) round cake pan and line the bottom with parchment paper.

Cream the butter and sugar: Beat the softened butter and 3/4 cup (150 g) granulated sugar on medium speed for 2–3 minutes, until pale and creamy. Scrape down the sides of the bowl as needed.

Add the eggs and vanilla: Beat in the eggs one at a time, allowing each to blend in before adding the next. Mix in the vanilla. The mixture may look slightly curdled at this stage, which is fine.

Combine the dry ingredients: In a separate bowl, whisk together the flour, baking powder, cinnamon, and kosher salt.

Finish the batter: Add the dry ingredients to the butter mixture in two additions, alternating with the milk. Mix on low speed just until combined. The finished batter should be thick and smooth. Do not overmix.

Spread the batter: Transfer the batter to the prepared pan and smooth the surface with a spatula. Because the batter is thick, spread it evenly all the way to the edges.

Add the plums: Arrange the sliced plums over the batter in slightly overlapping rows or a simple circular pattern. Gently press them into the batter without completely submerging them.

Add the cinnamon sugar: Mix the 1 tablespoon (12 g) granulated sugar with the 1/4 teaspoon cinnamon and sprinkle evenly over the plums.

Bake: Bake for 45–50 minutes, or until the top is golden brown and a toothpick inserted into the cake portion comes out with a few moist crumbs but no wet batter. The plum juices may leave the toothpick slightly moist if it enters a piece of fruit, so test the cake in a few places.

Cool before slicing: Let the cake cool in the pan for 15 minutes, then loosen the edges and transfer it to a wire rack. Cool for another 5 minutes before slicing. The cake should feel set in the center while remaining tender.