Preheat the oven to 175°C.
Grease a 22 cm spring-form cake tin.
Mix the flour, baking powder and salt. Using an electric mixer beat the butter and sugar until creamy, then add the eggs one at a time and keep beating. Then add the lemon juice and zest, combine well. Slowly add the flour to it whilst the mixer is running.
Transfer the smooth batter into the cake tin, spreading out evenly.
Cut the plums in half, remove the stones, then cut the flesh into thin wedges. Press this halfway into the batter, creating circles on the cake, then sprinkle with cinnamon sugar.
Bake in the oven for 45-50 minutes until golden brown and a skewer inserted in the middle comes out clean.
Turn off the oven, and with the door is opened, rest the cake in the oven for 5 minutes.
Remove the tin from the oven, release the sides and let the cake cool.
Serve warm or at room temperature.