Step 1

Preheat the oven to 350°F. Grease an 8" or 9" springform or cake pan, and line the bottom with a parchment round if using a cake pan.

Step 2

Cream the butter and sugar until light and fluffy. Scrape the bowl. Add the eggs and vanilla and beat until smooth and light. Scrape the bowl again.

Step 3

Add the salt, baking powder, and flour, and mix just until fully combined.

Step 4

Pour the batter into the cake pan. Arrange the plum halves, skin side up, on top of the cake. Dust with cinnamon.

Step 5

Bake approximately one hour, or until a cake tester comes out of the cake batter portion clean, and the plums are juicy and bubbling.

Step 6

Let fully cool before releasing the cake from the springform pan or turning out onto a dish. PRO TIP: A springform pan is preferable because the batter is a bit delicate. But if you are turning this cake out from a regular cake pan, never fear. Place a cooling rack directly over the cake pan, then in one swift, meaningful motion, flip the cake onto the rack. Remove the parchment. Then immediately flip it right side up again onto the serving plate.