1. Preheat oven to 350 degrees. Grease bottom and sides of a 9 or 25 cm springform pan.
2. Mix together flour, baking powder, and salt.
3. Beat together butter, sugar, and lemon zest at medium speed until light and fluffy. Add eggs, 1 at a time, beating well after each addition.
4. At low speed, beat flour mixture into butter mixture. Spoon batter into prepared pan, spreading evenly.
5. Arrange plum halves, skinsides up, on top of batter. Sprinkle with lemon juice.
6. To prepare topping, mix together sugar and cinnamon. Sprinkle over top of cake.
7. Bake cake until golden brown, 1 hour. Transfer pan to a wire rack to cool slightly. Recipe By : Great American Home Baking