Untested - Nectarine, Plum, and Apricot Upside-Down Cakes

http://www.marthastewart.com/353366/nectarine-plum-and-apricot-upside-down-cakes

For the Fruit Enhancer
 1 cup (2 sticks) unsalted butter, room temperature 
 3 tablespoons maple syrup 
 1 teaspoon dark rum 
 2 cups light-brown sugar, packed 
 1/2 teaspoon vanilla extract 
 1/2 teaspoon coarse salt 

For the Cake

 2 1/2 pounds nectarines, plums, or apricots, about 10 to 15 

 1 1/2 cups all-purpose flour, plus more for pans 

 1 1/2 cups cake flour, not self-rising 

 1 tablespoon baking powder 

 1 teaspoon salt 

 1 cup (2 sticks) unsalted butter, room temperature, plus more for pans 

 1 3/4 cups sugar, plus more for sprinkling fruit 

 4 large eggs 

 2 teaspoons pure vanilla extract 

 1 1/4 cups milk

1. Preheat oven to 350 degrees. Make the fruit enhancer: In the bowl of a stand mixer fitted with the paddle attachment, combine the butter, maple syrup, dark rum, light-brown sugar, vanilla extract, and salt until well blended.
2. Spray five 6-by-3-inch round cake pans or three 12-cup standard muffin tins with cooking spray; if using cake pans, line the bottoms with parchment paper and spray with cooking spray. Divide fruit enhancer evenly among cake pans or muffin tins and spread with an offset spatula to make smooth.
3. Slice fruit into 1/4-inch wedges. Starting from the inside and working outwards, arrange fruit slices in a fanlike, circular pattern on top of fruit enhancer, using about 2 to 3 pieces fruit per cake. If making mini upside-down cakes, slice fruit into circular, cross-sectional slices about 1/4-inch thick, using one round slice per muffin tin. You can also use thin wedges or slices for the mini upside-down cakes, and arrange in a decorative fashion.
4. Make cake: Into a medium bowl, sift together flours, baking powder, and salt; set aside.
5. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy, 3 to 4 minutes, scraping down the sides of the bowl as needed. Beat in eggs, one at a time, and then beat in vanilla. With the mixer on low speed, add the flour mixture in three parts, alternating with the milk and beginning and ending with the flour; beat until combined after each addition.
6. Divide the batter between the prepared pans, and smooth with an offset spatula. Bake, rotating the pans halfway through, until the cakes are golden brown and a cake tester inserted in the centers comes out clean, 35 to 40 minutes for the 6-inch cakes, or 20 to 25 minutes for the mini upside-down cakes. Transfer pans to a wire rack to cool 30 minutes, or 20 minutes for minis. Loosen side of cake with small offset spatula or paring knife. Invert cakes onto a rack set atop a baking sheet; peel off the parchment if necessary. Serve warm or cool.