Sweet Potato Sonker with Milk Dip

Ingredients:
4 cups all-purpose flour
1 tablespoon sugar
2 teaspoons fine sea salt
1 1/2 cups (3 sticks) unsalted butter, cold, cut into slices
1 large egg
1/2 cup ice-cold water
1 tablespoon apple cider vinegar
8 tablespoons (1 stick) unsalted butter, at room temperature, plus more for greasing the baking dish
1/3 cup all-purpose flour, plus more for dusting
6 medium (about 3 pounds) sweet potatoes, peeled
2 teaspoons salt
1 1/2 cups sugar
1 cup sorghum syrup
3 cups whole milk
2 tablespoons cornstarch
1 teaspoon vanilla extract

Instructions:
Whisk the flour, sugar, and salt together in a large mixing bowl. Using a pastry blender or fork and knife, cut in the butter. Make sure pea-sized butter chunks remain to help keep the crust flaky.
Lightly beat the egg in a medium-sized bowl. Whisk in the ice-cold water and the vinegar.
Pour the liquid mixture into the flour-butter mixture, and combine using a wooden spoon. Mix until the dough comes together in a shaggy mass. Be careful not to overmix. Use floured hands to divide the dough in half and then form into 2 balls. Wrap each ball tightly in plastic wrap. Let them chill in the refrigerator for at least 1 hour before rolling out and beginning the sonker recipe.
Butter and lightly flour a 13x9-inch baking dish (see Note).
On a floured surface, roll out one chilled dough ball into a large rectangle that will fit into the baking dish. Transfer the rolled-out dough to the prepared baking dish, and press it down gently to line the dish and form the bottom crust. Place the dish in the refrigerator to chill.
Put the whole peeled sweet potatoes in a large pot, add cold water to cover, and add the salt. Place the pot over medium heat, cover, and bring to a boil. Then reduce the heat to a simmer and cook until the potatoes are fork-tender, about 25 minutes.
Use a large slotted spoon to transfer the cooked potatoes to a cutting board to cool. Measure out and reserve 1 1/2 cups of the cooking liquid to use later. Slice the cooled sweet potatoes into rounds, making them as thin as possible without breaking them.
Preheat the oven to 375°F.
Remove the dough-lined pan from the refrigerator and layer the sliced sweet potatoes on top of the crust. In a medium bowl, combine 1 cup of the sugar, the sorghum, the 1/3 cup flour, the butter, and the 1½ cups reserved cooking liquid. Mix well and pour over the sweet potatoes.
Roll out the remaining dough ball into a rectangle a little smaller than the first. Cut it into strips that are about 1/2 inch wide, and form a lattice crust on top of the sweet potatoes.
Bake for about 40 minutes, until the crust is golden brown (the sonker will not be fully baked at this point).
While the sonker is baking, prepare the milk dip: Whisk 1/2 cup of the milk with the cornstarch in a medium saucepan, making sure all the cornstarch is dissolved. Add the remaining 2 1/2 cups milk and the remaining 1/2 cup sugar. Set the pan over medium-high heat and let it come to a boil. Let boil for 1 minute to thicken. Then remove from the heat and stir in the vanilla.
When the sonker has cooked for 40 minutes, pour 2 cups of the prepared milk dip over the entire surface. Return the sonker to the oven and bake for 15 minutes more or until it is caramelized around the edges and brown on top. Remove the dish from the oven and let it cool for at least 20 minutes before serving; the milk will continue to be absorbed and thicken.
Serve the sonker just warm, with the remaining milk dip on the side for drizzling.