Preheat oven to 350°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.
3/4 cup granulated sugar, 1/2 cup salted butter, 2 large eggs, 1 teaspoon lemon zest, 1 teaspoon vanilla extract
Add the flour, baking power, and salt, mixing just until combined, then spread the thick batter into the prepared pan.
1 1/4 cups all-purpose flour, 1 teaspoon baking powder, 1/4 teaspoon salt
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.
6-12 plums*
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.
1 cup all-purpose flour, 1 cup sugar, 1/4 teaspoon cinnamon, 1/2 cup salted butter
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).