Heat the oven to 350F. Grease a 9-inch round cake pan and place on a baking sheet. Please see tips above for the best kind of cake pan to use.
Cut the plums into thick slices. Sprinkle brown sugar on the bottom of the cake pan and arrange the plums on the bottom in a single layer. Melt the butter and pour over the plums. Bake in the oven for 10 mins.

In a bowl or stand mixer cream the butter and sugar together until light and fluffy. Separate the egg yolks from the whites and add the yolks one at a time till fully combined. Add the buttermilk, vanilla, lemon zest and mix till fully combined. In a bowl whisk all the dry ingredients together. In a seperate bowl whisk the egg whites with an electric mixer until soft peaks form. Add the dry ingredients to the wet ingredients and mix together till just combined. Then slowly fold the egg whites into the batter until just combined.

Pour the batter over the plums and bake until the the cake is lightly browned and a toothpick inserted in the center comes out clean. Begin checking for doneness at 30mins. Let the cake cool slightly, then place a serving plate over the top and carefully turn over so the plums are on top.
Best served with yoghurt and a sprinkle of pistachios.