Preheat oven to 425°. Place oven rack in bottom third of oven.
In a large bowl, combine apples, cranberries, sugars, cornstarch, lemon juice, vanilla, pie spice, and salt.
On a lightly floured surface, roll half of Buttermilk Pie Dough to a 12-inch circle. Transfer to a 9-inch deep-dish cast-iron pie plate, pressing into bottom and up sides. Spoon apple mixture into dough and dot with cubed butter.
On a lightly floured surface, roll remaining Buttermilk Pie Dough to a 12-inch circle. Cut vents in dough with a small cookie cutter. Place dough on top of filling. Pinch top and bottom edges of crust together. Trim to ½-inch past edge of pie plate, if necessary. Fold edges under, and crimp as desired. Freeze until firm, about 15 minutes.
Place pie plate on a rimmed baking sheet. Brush crust with lightly beaten egg.
Place in oven and immediately reduce oven temperature to 375°. Bake until golden brown and filling bubbles in center, 1 hour and 10 minutes to 1 hour and 20 minutes. Let cool for at least 30 minutes. Serve warm or at room temperature.
Buttermilk Pie Dough
In the work bowl of a food processor, combine flour and salt; pulse until combined. Add cold butter; pulse until mixture is crumbly and butter is in pea-size pieces. With processor running, add 8 tablespoons cold buttermilk in a slow, steady stream just until dough comes together. (Mixture may appear crumbly, but it should be moist and hold together when pinched.) Add additional buttermilk, 1 tablespoon at a time, if needed.
Turn out dough and divide in half. Shape each half into a disk. Wrap each disk tightly in plastic wrap and refrigerate for at least 30 minutes. Dough may be refrigerated for up to 3 days or frozen for up to 2 months.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/cranberry-apple-pie/