Preheat the oven to 350 degrees F. In a medium bowl, mix together the flour, baking powder, baking soda and salt. 

Next, in the bowl of a stand-up mixer with the paddle attachment, cream together the butter and sugar until light and fluffy, about 2 minutes. Add the eggs and vanilla, and beat until thoroughly incorporated. 

Add half of the flour mixture and all of the buttermilk into the butter and sugar mixture. Then stir in the remaining flour mixture, being sure not to over mix. Add the pistachios and mix one last time. 

In a seasoned 10” cast iron skillet, melt the butter over medium heat. Add the brown sugar and salt, and mix until the sugar begins to bubble, about 1 minute. Turn off the heat and let stand for a few minutes to slightly cool. 

Arrange the slices of plums and nectarines in a circular pattern, overlapping a few of them, on top of the brown sugar mixture. Pour the cake batter over the slices of fruit, and spread the batter gently so it’s nice and even. 

Transfer the skillet to the oven and bake for 40 to 45 minutes, or until a wooden skewer comes out clean. 

Remove the cake from the oven and allow to cool on a wire rack for 5 to 10 minutes. To invert the cake, run a knife along the outside edge of the cake, place a plate on top of the cast iron skillet, and turn both of them over—be careful, they’ll be heavy! Lift up the skillet and there you have it! Your cake! Serve this cake slightly warm with a dollop of whipped cream.