Fresh Plum Cake, also called Pflaumenkuchen or German Plum Cake, is a thin, rustic sheet cake baked with fresh plum slices on it. Simple, and not too sweet.
Slice and pit your plums, and set them aside.
Line a jelly roll pan (no larger than 10"x15"x2") with parchment, or grease/spray well.
Preheat oven to 350° F.
In a mixing bow, cream together sugar, butter, and lemon zest.
Beat in 1 egg at a time.
Add the flour and baking powder and mix together by hand, using a spoon.
Add 1 tsp milk and mix just until incorporated. Don't over mix.
Spread the batter on the pan using a spatula. Dip the spatula in water to help with the spreading (don't saturate the batter though). This will be a thin layer of batter, don't worry as this is correct.
Arrange the sliced plums on top of the batter, evenly spaced, with the skin side down (this keeps most of the juice in the fruit, not the batter).
Sprinkle a layer of sugar over the top of the fruit and exposed batter (I end up using 3 to 6 Tablespoons. It depends on how sweet the plums are).
To make the topping, combine flour and sugar in a separate bowl. Cut in the cold butter using a pastry cutter (or you can grate it over the bowl) and mix to make crumbs. Sprinkle over top of the cake.
Bake for 20 to 30 minutes, or until a toothpick inserted into the batter comes out clean. You may need to bake about 10-15 minutes longer if adding the streusel topping.
Cool the cake on a wire rack and then slice as you wish and transfer the pieces to a serving dish.
This cake should be eaten within a day, though it does freeze well.