Pluot-blueberry upside-down cake

Butter, unsalted	3 tablespoons + ½ cup
Sugar, light brown	¾ cup
Blueberries	6 ounces
Pluots, halved, pitted, cut into ½-inch slices	1 pound
Flour, all-purpose	1 ½ cups
Baking powder	1 ½ teaspoons
Salt	¼ teaspoon
Sugar, granulated	¾ cup
Vanilla extract	1 teaspoon
Eggs, room temperature	2 large
Mile, whole, room temperature	½ cup

1. Preheat the oven to 350°F. 
2. To make the topping, in a 9-inch round cake pan or a cast-iron skillet, melt 3-tablespoons butter over low heat. 
3. Add the brown sugar, and stir until thoroughly moistened; remove from heat, and let cool briefly. 
4. Distribute half of the blueberries evenly over the brown sugar mixture. 
5. Arrange the pluot slices over the blueberries in overlapping concentric circles. 
6. Strew the remaining blueberries on top of the pluots; set aside. 
7. To make the cake, in a small bowl, whisk together the flour, baking powder, and salt. 
8. In a stand mixer fitted with the paddle attachment, beat together ½-cup butter and granulated sugar on medium speed until the mixture is light and fluffy, about 3 to 5 minutes. 
9. Add the vanilla and eggs, one at a time, beating until completely incorporated. 
10. Gradually mix in half of the flour mixture. 
11. Stir in the milk, followed by the rest of the flour mixture, and mix until combined. 
12. Scrape the batter on top of the fruit, and smooth it into an even layer. 
13. Bake until golden brown and a toothpick inserted in the center of the cake comes out clean, about 1 hour. 
14. Let cool for 15 minutes. 
15. Run a knife around the sides of the cake to help loosen it from the pan. 
16. Invert a serving plate over the pan. 
17. Wearing oven mitts, grasp both the pan and the plate, and turn them over together. 
18. Carefully lift the pan. 
19. Serve the cake warm with whipped cream or ice cream. 

Recipe mildly adapted from Ready for Dessert by David Lebovitz.