No two bakers of stack cake go about it the same way, which means there's no definitive stack-cake recipe or technique, only a common concept and love for the simple dessert. This is the version Appalachian writer and cookbook author Sheri Castle has developed over three decades, and the recipe she plans to pass along to her daughter.
To make the filling: Place the apples, brown sugar, and spices in a large saucepan. Add enough of the water to cover. Bring to a boil over high heat, then reduce the heat to low and let simmer, stirring frequently, until the apples are tender and the filling is very thick, about 1 hour. If the mixture gets dry, add more water. If it's soupy, continue to simmer until the excess liquid cooks away. Remove from the heat and use a potato masher to break up the apples into a chunky sauce. Stir in the boiled cider. The finished consistency should be similar to apple butter.
To make the cake: Preheat the oven to 350°F. Grease and flour five 9" cake pans. If you don't have that many pans, bake the batter in batches.
In a large bowl, whisk together the flour, sugar, baking soda, baking powder, and salt. Add the sorghum, 1/2 cup of the buttermilk, shortening, and egg. Beat with an electric mixer on low speed until the mixture is smooth, with the consistency of cookie dough. If the dough is very dry and stiff, add the remaining 2 tablespoons buttermilk.
Divide the dough into five equal pieces (they'll weigh between 7 and 7 1/2 ounces each).
Wrap each piece in plastic so it won't dry out. Use your lightly floured hands to pat a piece of dough evenly into the bottom of each of the prepared cake pans. The dough should be about 3/8" thick. Lightly prick the dough all over with a fork.
Bake the layers until firm when lightly pressed, about 15 minutes. The layers won't rise much as they bake. Remove the layers from the oven, and assemble the cake immediately, without letting them cool.
To assemble the cake: Turn the first layer out onto a large cake plate. Immediately spread it with 1/4 of the apple filling (about 1 heaping cup). Continue stacking and topping the warm layers. Leave the top layer bare.
Cover the cake with several layers of plastic wrap and then tea towels, or store it in an airtight cake carrier. Let the cake rest at room temperature for at least 2 days before dusting the top with confectioners' sugar, cutting, and serving.
