Preheat oven to 180 C and butter a 9 inch pan (springform preferably).
Halve plums lengthways and remove seeds. Add room temperature butter and ¾ cup sugar to a bowl and cream them together with an electric mixer until light and fluffy.
Add eggs one at a time, whisking after each addition. Scrape the sides of the bowl. Sift the baking powder, flour and salt into the bowl and whisk for a few seconds, just until the batter comes together, no more. Don’t over-whisk.
Spoon the batter into the greased pan. Arrange the plums face down over the top, making sure to cover the entire surface. Mix 2 tsp sugar and 1 tsp cinnamon together and sprinkle over the plums.
Bake for 55 - 60 minutes or until cake is golden and a toothpick inserted into the thickest part comes out clean. Cool for at least an hour before releasing the sides. Can be stored at room temperature, covered, for up to 2 days.