For the crust
6 oz. (1-1/3 cups) unbleached all-purpose flour; more for rolling
4 oz. (1/2 cup) cold unsalted butter, cut into small cubes
1 tsp. granulated sugar
1/2 tsp. table salt
1 large egg yolk, lightly beaten
For the filling
2 lb. medium parsnips, peeled, cored, and cut into large chunks
1-1/2 cups buttermilk
2/3 cup packed dark brown sugar
2 large eggs
1/2 tsp. table salt
1/2 tsp. ground cinnamon
1/2 tsp. ground ginger
1/4 tsp. freshly grated nutmeg
1/8 tsp. ground cloves
Make the crust
In a stand mixer fitted with the paddle attachment, combine the flour, butter, sugar, and salt and mix on medium speed until the butter blends into the flour and the mixture resembles a coarse meal, about 2 minutes. Mix 2 Tbs. ice-cold water with the egg yolk in a small bowl. With the mixer on low speed, add the yolk mixture and mix until just combined. Transfer the dough to a work surface and bring it together with your hands. Shape it into a 1-inch-thick disk, wrap in plastic, and refrigerate for at least 1 hour.
Set the dough on a lightly floured surface, sprinkle a little flour over it, and roll it out into a 1/8-inch-thick circle that’s about 12 inches in diameter, reflouring the dough and work surface as necessary.
Transfer the dough to a 9-inch pie plate and gently fit it into the pan, lifting the edges and pressing the dough into the corners with your fingers. Trim the edges, leaving a 1/2-inch overhang. Fold the overhanging dough underneath itself and crimp the edges. Prick the dough all over with a fork. Refrigerate for at least 1 hour.
Position a rack in the center of the oven and heat the oven to 425°F.
Line the dough with foil or parchment, fill with dried beans or pie weights, and bake for 15 minutes. Reduce the oven temperature to 350°F. Carefully remove the foil and weights and continue to bake until the bottom looks dry and the edges are light golden, an additional 5 to 8 minutes. Cool completely before filling (leave the oven at 350°F).
Make the filling
Bring a large pot of water to a boil. Add the parsnips and cook until tender when pierced with a fork, 12 to 15 minutes. Drain the parsnips in a colander and let them steam under a clean kitchen towel for about 5 minutes. Return the parsnips to the pot and mash them with a potato masher, keeping the mixture rather rough. Measure 2 cups of the parsnip mash; save any extra for another use.
Purée the 2 cups of mashed parsnips and the buttermilk in a blender until smooth. Transfer the purée to a mixing bowl. With a whisk, beat in the sugar, eggs, salt, cinnamon, ginger, nutmeg, and cloves, whisking until the sugar dissolves.
Bake the pie
Pour the filling into the pie crust and bake until the top is lightly browned and a toothpick inserted in the center of the filling comes out clean, about 1 hour. Cool on a rack for at least 1 hour.
Serve at room temperature.