Preheat the oven to 350F (177C). Line a 9" (23cm) springform pan with parchment paper, along the bottom. Lightly grease the sides of the pan. Set aside. If making your own buttermilk, add 2 teaspoon lemon juice to the ½ c room temperature whole milk now. It needs about 10 minutes to curdle. In a medium size mixing bowl, whisk together the cake flour, all purpose flour, baking soda, baking powder, ground cardamom, ground cinnamon and salt. Set aside.
Zest the lemon into a small bowl filled with sugar. Rub the sugar and zest together until a wet sand like texture forms.
Using a whisk (by hand) or electric mixer with beaters or paddle attachment, beat together the oil and butter together until smooth and well combined.
With the mixer on low, slowly stream in the lemon sugar into the oil butter mixture untill all the sugar is added. Increase the speed to mediumm, beating until the mixture is light and fluffy, about 2-3 minutes.
Add eggs and vanilla extract, beating each one for about 20 seconds on medium speed before adding the next.
Stir in the dry ingredients and buttermilk to the wet, alternating in this order: dry, buttermilk, dry, buttermilk, dry. Leave a few flour or milk streaks before adding the next .
Pour batter into the prepared pan. Smooth the top of the batter. Top with sliced plums.
Top the batter with the chilled streusel topping. Feel free to clump pieces together for larger crumbles or scatter smaller pieces over the top of the batter. Gently press the struesel into the cake batter and plums. Bake at 350 F (177C) for 50-60 minutes. Around 45 minutes, do the first check of the cake. Check to make sure the struesel isn't burning. If the top is browning too much, place a tent foil over the top for the remainder of the time. Bake an additional 5 minutes before checking for doneness. The edges of the cake will have pulled slighly from the pan and a toothpick inserted into the center will come out clean or with a few moist crumbs. If the toothpick comes out wet or "gummy," check every 5-10 until done. Remove the plum cake from the oven to cool in the pan for 30 minutes.
Cool the cake an additional 30-45 minutes on the cooling rack if desired. Top with a dusting of powdered sugar, slice and enjoy!