Plum Cornmeal Upside-Down Cake

8 Tbsp. (1 stick) unsalted butter, divided

1/3 cup brown sugar, packed

1 pound plums, halved, pitted

1/4 tsp. ground cinnamon

Pinch of sea salt

1 cup all-purpose flour

1/2 cup cornmeal

1 tsp. baking powder

1/2 tsp. sea salt

1/8 tsp. baking soda

3/4 cup granulated sugar

1/2 cup coconut milk (or whole milk)

2 eggs 

1/4 cup fresh-squeezed lemon juice

2 tsp. lemon zest

1. Preheat oven to 350 degrees F. 
2. In an 8-inch non-stick (or cast iron) skillet over medium heat, melt 2 Tbsp. unsalted butter with 1/3 cup brown sugar, 1/4 tsp. cinnamon, and a pinch of sea salt. Cook for about 3 minutes, stirring frequently, until sugar is dissolved. 
3. Remove skillet from heat and arrange the halved, pitted plums cut-side up in the butter-sugar mixture. Set aside. 
4. In a large mixing bowl, combine the dry ingredients: all purpose flour, cornmeal, baking powder, sea salt, and baking soda. Whisk to combine, and set aside. 
5. In a medium bowl, combine the wet ingredients: granulated sugar, milk, eggs, lemon juice, and lemon zest. Whisk until well combined. 
6. Pour wet ingredients into the dry ingredients, and mix until combined and no dry bits remain. 
7. Pour batter evenly over the plums in the skillet. 
8. Bake the cake in the skillet at 350 degrees F for 35-40 minutes, or until a toothpick inserted in the center of the cake comes out clean. 
9. Remove the skillet from the oven, and allow to cool for 15 minutes. Then, carefully run a dinner knife around the edge of the skillet to loosen the cake from the pan. Place a large plate over the skillet, and carefully invert the skillet to release the cake onto the plate. If some sticky bits remain in the pan, you can scrape them out and spread them back on the top of the cake. 
10. Allow the cake to cool completely, then slice and serve!