Preheat oven to 350 degrees F (177C). Grease a 9-inch springform cake pan - Line with baking paper.
In the bowl of a stand mixer (paddle attachment), mix flour, baking powder. Add sugar, vanilla extract, lemon zest, butter and eggs and beat on low speed until combined, then increase speed to medium for 2 minutes until smooth and creamy. Do not overmix
Spread the thick batter evenly in the prepared cake pan. Place the plums on top.
Streusel topping: In a bowl, combine flour, sugar, cinnamon. Add the soft butter and mix together with a silicone paddle until combined, with a coarse crumble.
With your fingers, sprinkle the Streusel crumbs evenly over the fruits and batter.
Bake for 50 minutes until the top is golden. Remove and let cool for 10 minutes before removing the outer ring from the springform pan. Can be enjoyed warm or at room temperature.