Preheat oven to 350 degrees F.
Arrange the plums in a single layer on the bottom of a spring form pan. Sprinkle the plums with 1/4 cup of the brown sugar.
Melt the butter slowly over medium heat, until its smells nutty, swirling the pan occasionally. Butter will foam and froth then turn brown, with brown bits sticking to the botton of the pan. Pour browned butter, including bits at the botton of the pan, over the plums. Place plums in oven for 10 minutes.
Meanwhile, whisk all your dry ingredients: flour(s), cornmeal, baking powder and salt. Set aside.
In another bowl whisk all the wet ingredients together, except for the egg whites. Thats the egg yolks, olive oil, yogurt, vanilla and lemon zest. Add the wet ingredients to the dry ingredients and mix together well.
Whisk the egg whites with an electric mixer until soft peaks form. Slowly fold the egg whites into the batter until just combined.
Pour the batter over the plums, trying to spread in an even(ish) layer. Bake about 50-55min, until the the cake is lightly browned and a toothpick inserted in the center comes out clean.
Let the cake cool just slightly, then place a serving plate over the top and carefully turn over so the plums are on top.
For the maple vanilla yogrut: mix these three ingredients together. Add a dollop on the cake. Top with pistachios. Can serve with remaining yogurt. Enjoy warm or at room temp.