Start the filling first. Put the apples in a Dutch oven, and add enough water to cover the apples by 2 inches. Bring to a boil over high; then turn down the heat to low. Simmer, stirring occasionally, until apples are tender enough to mash, 1 to 1 1⁄2 hours. You may need to add a little more water to keep the apples from sticking, but you don’t want the final mixture to be soupy.

While the filling continues to cook, prepare the skillet for baking, and make the dough. Liberally grease the inside of a 9-inch cast-iron skillet with shortening, and sprinkle with 1 tablespoon of the flour, shaking and turning to coat the bottom of the skillet and about 1 inch up the sides.

Sift 5 1⁄2 cups of the flour with baking soda and salt, and set aside. Place the butter and granulated sugar in a large bowl, and beat with an electric mixer on medium speed until blended, about 3 minutes. Add the sorghum syrup, and beat 1 minute to blend. Whisk together the buttermilk and the eggs in a small bowl. (This recipe does not use baking powder, so buttermilk is necessary to activate the baking soda.)

Add flour mixture to the butter mixture alternately with the egg mixture in 5 additions, beginning and ending with flour mixture. Beat on low speed after each addition to incorporate. Cover and chill at least 1 hour or up to 2 hours.

After the apples are cooked, stir the brown sugar and mace into the apples until sugar is dissolved. Remove from heat, and mash with a potato masher until a thick puree forms with lumps no larger than a pea. (You can also pulse the mixture in the bowl of a food processor 15 times.) If mixture is too runny, return to medium heat, and continue to cook, stirring constantly, until liquid evaporates and mixture is the consistency of apple butter. Cover to keep warm.

Preheat oven to 350°F. Generously sprinkle a flat surface with some of the remaining flour, and scrape the chilled dough onto it. With floured hands, shape dough into a log about 10 inches long. Using a sharp, floured knife, cut into 5 (2-inch) pieces. With floured hands, roll each piece lightly in the flour, and shape into a ball. Place 4 of the balls on a rimmed baking sheet lined with parchment paper, cover with plastic wrap, and chill until ready to use.

Pat remaining 1 ball into a disk. Place dough disk in the greased and floured skillet and, pressing lightly with floured palm and fingers, flatten it evenly so it spreads out to just touch the edges of the skillet all around. Don’t pat too hard and don’t press it up against the side of the skillet, or it will stick. Evenly prick the surface of the dough lightly with a fork.

Bake in preheated oven until the top is golden and the cake has pulled slightly away from the edges of the skillet, about 25 to 30 minutes. It will not rise like a normal cake layer but will look like a big cookie, only much more tender. Let cool in skillet on wire rack for 5 minutes. Run a butter knife around the inside edge of the skillet, and gently nudge the layer underneath to loosen. Turn cake layer out onto a rack, and allow to cool for about 5 minutes. Transfer layer to a cake plate, and spread warm apple mixture over the top, to the edges. The apple filling should be about 1⁄4 inch thick, and you should use about 3⁄4 cup of the mixture per layer.

Bake in preheated oven until the top is golden and the cake has pulled slightly away from the edges of the skillet, about 25 to 30 minutes. It will not rise like a normal cake layer but will look like a big cookie, only much more tender. Let cool in skillet on wire rack for 5 minutes. Run a butter knife around the inside edge of the skillet, and gently nudge the layer underneath to loosen. Turn cake layer out onto a rack, and allow to cool for about 5 minutes. Transfer layer to a cake plate, and spread warm apple mixture over the top, to the edges. The apple filling should be about 1⁄4 inch thick, and you should use about 3⁄4 cup of the mixture per layer.

Allow cake to cool completely, about 30 minutes. Wrap cake in cheesecloth, and then wrap in several layers of plastic wrap. Allow to “ripen” at room temperature for 2 to 3 days before cutting and serving. Dust the top of the cake with powdered sugar, if desired.