Preheat the oven to 350 degrees.

Cut each plum in half (or quarters) and discard the pit. Set aside the plum sections.

Cream the butter and 1 cup of sugar using a stand mixer until light and fluffy. Add the room temperature eggs, one at a time, beating well after each addition.

Sift the flour, salt, and baking powder into a metal bowl.

With the mixer on low speed, gradually spoon the flour into the batter. Mix for 30 seconds to incorporate the ingredients.

Lightly grease the bottom of a 9-inch spring form pan. Pour the batter into the pan and use an offset spatula to distribute the batter evenly.

Place the plums evenly across the surface, skin side up.

Combine one tablespoon of sugar and one teaspoon of cinnamon and sprinkle the mixture evenly over the plumbs using a small wire sieve. Drip the lemon juice on the top – as evenly as possible.

Bake for one hour or until a cake tester comes out clean. Let the cake cool for 5 minutes. Run a small knife around the edge to release the cake from the sides of the spring form pan. Release the latch and remove the spring form collar. The cake can be served warm, or at room temperature. This cake ages nicely and can be made the day ahead.