Preheat oven to 350ºF. Prepare a 9 or 10″ springform pan and set aside.

Cream the butter and sugar in a mixing bowl until pale and fluffy. Add the flour, baking powder, eggs and salt and mix on low speed. Once flour is incorporated increase speed to medium and mix well.

Spoon the batter into the baking pan. Arrange the plum halves skin side down in the batter. Mix the cinnamon and sugar together in a small bowl and sprinkle it over the top of the plums.

Bake for 45-50 minutes, or until cake test inserted in the center comes out clean. Remove from oven and allow to cool before removing from pan and serving.