• 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).

Make the Topping

• 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.