• Preheat the oven to 350 °F. Line the bottom of an 8" square or 9 round cake pan with parchment paper.

Caramel Topping

• Combine1 ½ oz unsalted butter, ¾ oz dark rum, 6 oz light brown sugar, ¼ vanilla bean½ teaspoon ground cinnamon in a small saucepan and heat until the butter is melted and the mixture just begins to boil. Pour the warm caramel into the prepared pan and set aside while preparing the apples.
• Peel core and thinly slice 2 large apples. Place the apple slices in overlapping rows onto the caramel. Set aside while preparing the batter

Batter

• Combine 1 large egg, 1 large egg yolk and half of the buttermilk in a small bowl and set aside.
• In a mixing bowl, sift together6 ¾ oz cake flour, 1 teaspoon ground cinnamon, 1 ½ teaspoons baking powder, and ¼ teaspoon salt. Add 6 oz granulated sugar. Run the mixer on low speed for 30 seconds to combine the dry ingredients. Add 4 oz butter to the dry ingredients and mix until combined. Add the remaining buttermilk to the flour and butter mixture. Turn the mixer to medium high for 2-3 minutes to aerate the batter.
• Add the egg mixture in two batches, scraping the bowl and mixer between additions. Spread the batter over the apples.
• Bake until the center of the cake springs back lightly when pressed, or a toothpick inserted in the center comes out clean, about 30 minutes.
• Cool the cake in the pan for 5 minutes. While the cake is still slightly warm, run a paring knife around the sides of the and and invert it onto a serving platter
• If any caramel or apples stick in the pan scrape them out with a small spatula and place them on the cake while it's still warm.
• Serve warm or room temperature