Wash and slice the plums in half. If plums are bigger you might want to quarter them. Put them to the side.
Grease a 9 inch springform with butter and preheat oven to 350°F
Cream the butter and sugar together in a mixing bowl until fluffy. Approx 2 minutes. Add eggs, lemon zest and vanilla. Mix for 1 minute.
Mix flour, baking powder and salt in a seperate bowl. Add flour mixture in batter and mix well. Spread the thick batter into the prepared pan.
Arrange plums in a circular pattern on top of the cake batter until the batter is covered completely.
For the streusel mix the sugar, cinnamon, soft butter and flour in a bowl. Mix until it starts clumping together. Take it out of the bowl and sprinkle it evenly over the plums.
Bake the cake for 50 to 55 minutes or until golden brown, and insert toothpick in center of cake to see if it comes out clean.
Let the cake cool down completely before slicing. Serve with whipping cream or a scoop of vanilla ice cream
Enjoy!