Preheat oven to 325° F.
Melt 6 tablespoons butter and brown sugar in cast iron skillet over medium-high heat, stirring occasionally. Continue to cook until sugar is dissolved and mixture is bubbling, about 2 minutes.
9 tablespoons unsalted butter, softened, ⅔ cup packed brown sugar
Remove from heat and add 3 tablespoons buttermilk, pecans, and ¼ teaspoon salt. Stir to combine. Lay apple slices on top of mixture in a pattern. Set pan aside.
1 cup, plus 3 tablespoons buttermilk, 1 cup pecan pieces, ⅜ teaspoon salt, 1 medium apple, thinly sliced
In a medium bowl, add flour, cinnamon, cloves, baking powder, baking soda, and ⅛ teaspoon salt. Stir with a whisk to combine. Set aside.
2 cups cake flour, 1 teaspoon cinnamon, ½ teaspoon ground cloves, ½ teaspoon baking powder, ½ teaspoon baking soda
In a separate medium sized bowl, add the sugar, 4 tablespoons butter, and vanilla. Beat with a mixer at high speed until well blended.
¾ cup sugar, 2 teaspoons vanilla extract
Add the flour mixture to the sugar mixture a little at a time alternating with the buttermilk, starting and ending with the flour mixture. Mix just until almost combined. Finish mixing by folding in any remaining dry ingredients with a rubber spatula, just until dry ingredients are incorporated.
Spread batter into prepared skillet.
Bake in the 325° F oven for 45-48 minutes or until cake begins to pull away from the sides of the pan.
Remove from the oven. Allow to cool for 5 minutes. Place serving platter over the top of the skillet and, using oven mitts, carefully flip the cake over. Gently lift the skillet and allow cake to cool for 10 more minutes before slicing and serving warm.