In an 8-inch nonstick skillet over low heat, combine the brown sugar, maple syrup, butter, 3/4 teaspoon cinnamon and a pinch kosher salt. Melt together and whisk to combine. Turn the heat up to medium and bring the mixture to a boil. Allow to boil vigorously for 2 to 3 minutes. Carefully add the apples and toss around until the apples are fully coated in the caramel. Reduce the heat to low and allow to bubble for 3 to 4 minutes, or until the apples begin to soften and appear translucent. Place the skillet onto a parchment-lined baking sheet (to catch any bubbled-over caramel) and bake for 5 minutes while you prepare the batter.
In a large 8-cup liquid measure or large mixing bowl, beat the eggs and granulated sugar until frothy. Add the buttermilk, flour, vanilla, 1/2 teaspoon cinnamon, 1/4 teaspoon kosher salt and the nutmeg if using. Whisk to combine until no longer lumpy.
Open the oven and carefully remove the pan. Pour the batter into the skillet over the apples, swirling around the perimeter and ending in the center. (Beginning on the sides and working inward gives the pancake better structure instead of just pouring all of the batter in the middle. This will also ensure that the pancake “souffles” evenly). Place back into the oven to bake for 18 to 20 minutes, or until the sides have risen and the edges are dark. Remove from the oven and allow to settle for a minute or two. Carefully invert on a plate. Sprinkle with flaky salt and serve immediately.