Upside Down Plum Cake

This cake is the perfect summer recipe.  It's my favorite mix of tart and sweet, and it's oh-so pretty.  It also takes advantage of the delicious, ubiquitous summer plum.  Adapted from Mom's Kitchen Handbook.  


 

6 to 8 plums, depending on size
1/4 cup brown sugar
1 stick unsalted butter (8 tablespoons), divided
1 cup + 2 1/2 tablespoons whole-wheat pastry flour
1/2 cup almond flour
2 teaspoons baking powder
1/4 teaspoon salt
11 tablespoons sugar
2 eggs, separated
1 cup + 3 tablespoons buttermilk
1 teaspoon vanilla extract

Preheat oven to 425 degrees. 
Cut plums in half and remove pits.  
Sprinkle brown sugar and 2 tablespoons of butter over the bottom of a 9-inch cake pan.  Place the pan on a burner to melt the butter and sugar together over low heat.  Spread evenly over the bottom of the pan.  
Place the plums flat-side down into the cake pan.  Arrange the halves so that the fruits are touching.  
Use an electric mixer to cream the remaining butter and sugar together.  Add the eggs and mix until well-blended.  Add buttermilk and vanilla and mix again.  
In another bowl, mix remaining dry ingredients (all but sugar) together.  
-Add the dry ingredients to the wet mixture.  Cream together until batter is smooth.  
Pour the batter over the plums in cake pan.  
Place in the oven with a baking sheet underneath to catch any spillage.  
Bake for about 33 minutes.  
Cool completely (at least 1 hour).  Flip cake by placing a plate over the pan, then flipping and lifting the pan off of the cake.  
Enjoy!