Preheat oven to 350 degrees.
Mix sugar and butter in a mixing bowl (preferably with an electric mixer).
Add four, baking powder, eggs, and almond extract. Beat until fully mixed.
If you are not using a non-stick spring form pan, spray cooking oil before pouring batter into the pan. Place the plums skin-side up on top of the batter so that they're touching each other.
Dust lightly with sugar, lemon juice and cinnamon (about 1 teaspoon).
Bake for about 45 minutes to an hour.
I enjoyed it both right out of the oven and the next day at room temperature--both were delightful!
It's best to use a spring form pan for this but you can also use a regular cake tin. I used an 8-inch spring form but a 9 and 10 inch also work here.