Preheat oven 400ºF and grease and line the base of a 9 inch springform cake pan
Whisk together the flour, baking powder and salt.
In the bowl of a stand mixer beat the butter until smooth.
Add the sugar and beat for 2 or 3 minutes. Add the lemon zest, egg and vanilla extract, beat well.
On low speed, alternately add flour mixture and milk on low speed (end with flour). Mix until just smooth.
Spread the batter over the base of the baking pan using the back of a spoon.
Arrange the plums decoratively over the dough.
Put flour, sugar, butter, cinnamon and salt into a bowl (it's fine to use the bowl you made the cake in without cleaning it). Rub the butter into the other ingredients forming a crumbly streusel.
Bake for 35 to 40 minutes until a skewer inserted comes out clean. Always adjust cooking time if your oven is hotter or cooler.
Allow to cool for 10 minutes before unclipping the sides of the springform pan and removing carefully. Cool for at least another 30 minutes.