Preheat oven to 350 degrees F. Grease and flour a 9-inch springform pan or line a 9-inch square pan with a parchment paper sling.

Cream the butter and sugar together in a large bowl until light, about 2 minutes. Add the eggs, lemon zest, and vanilla, mixing well to combine.

Add the flour, baking power, and salt, mixing just until combined, then spread the thick batter into the prepared pan.

Wash and slice the plums, then arrange them in a circular pattern on top of the cake batter, completely covering the batter and slightly overlapping.

Prepare the streusel by mixing the flour, sugar, cinnamon, and softened butter in a bowl using a hand mixer for 1-2 minutes until it resembles crumbs. It should clump together if you give it a squeeze in your fist to make larger clumps of streusel. Sprinkle evenly over the plums.

Bake for 45-55 minutes until golden brown and a toothpick inserted into the center of the cake comes out clean. Cool for at least 30 minutes before slicing and serving. This cake is delicious both warm with a scoop of ice cream or served cold from the fridge all by itself (my personal favorite).