Start by preparing the apple pie filling first so it has enough time to cool.

In a medium saucepan over medium-high heat, add the butter, diced apples, brown sugar, maple syrup, cinnamon, apple pie spice, and salt. Cook until the sugar melts and the apples start to soften, stirring occasionally (~10 minutes).

Mix together the cornstarch and water in a small bowl to create a slurry. Pour the slurry into the apple mixture, stir, and cook until the mixture thickens (1-2 more minutes).

Remove from the heat and stir in the vanilla. Set aside to cool completely.

Preheat the oven to 425F. Grease a 12" cast iron skillet or large round baking pan with butter. Set aside.

In a medium bowl, whisk together the buttermilk, Greek yogurt, and vanilla until well combined. Set aside.

In a separate large bowl, whisk together the cake flour, all-purpose flour, sugar, baking powder, baking soda, cinnamon, and salt. Add in the grated butter, and use a wooden spoon to toss until butter is coated in flour and the mixture resembles a 'crumbly' texture.

Make a well in the center of the dry mixture, and pour in the wet mixture. Mix with a wooden spoon until just combined, being careful not to over-mix.

Make a large dimple in the center of your dough, and place the apple pie filling inside the dimple. Use your hands to gently fold 3-4 times, until the apple pie filling is just evenly dispersed.

Use a large cookie scoop or a ½ cup measuring cup to scoop the dough for each biscuit, and place in the prepared skillet or pan.

Mix together the brown sugar and cinnamon in a small bowl for the cinnamon sugar topping. Once ready, brush the tops of the biscuits with buttermilk or heavy cream. Then, sprinkle the tops with the cinnamon sugar mixture.

Bake for 20-25 minutes, or until fluffy and golden brown. Remove the biscuits from the oven and let rest for at least 5-10 minutes. Then, serve them warm with butter, apple butter, flaky sea salt, and a hot cup of coffee, if desired.