In a large bowl, combine the flours, salt and sugar. Add the butter and rub with your fingertips until the mixture looks like gravel, with some butter worked in and some 1/4" chunks remaining. Gradually add the starter, folding the mixture with a spoon or your hands until it just starts to come together into large clumps, with no floury bits remaining. Gather the dough into a ball, flatten into an 8" disc, wrap in plastic and chill at least 30 minutes. (The dough will keep, refrigerated, for several days, or tuck into a ziploc bag and freeze for up to a couple of months.)
On a lightly floured surface, roll out the dough into a 12" round, flouring the top of the dough and the rolling pin as needed. Fit the dough into a 9" pie pan, trim the edge to a 1" overhang, and tuck it under. Flute the crust. Chill in the fridge for 30 minutes, then freeze for 20 minutes.
Meanwhile, position a rack in the bottom third of the oven and preheat to 400º. If you have a baking stone, set it on the rack.
Place the frozen crust on a rimmed baking sheet, place a round of parchment paper inside the crust, and line with pie weights (or dried beans or cleaned pennies - I keep mine in a cheesecloth bag for easy handling.)
Bake the crust until the bottom looks dry and light in color underneath the parchment, 20 - 30 minutes. Carefully remove the weights and continue baking the crust until light golden brown on the bottom, 5 more minutes.
For the filling:
Prick the potatoes several times with a fork. Place on a small sheet pan lined with parchment paper and roast in the oven at 400º until collapsing and oozing juices, 40 minutes to 1 hour. (They should be quite tender when squeezed with a pair of tongs.) Let cool until handleable, then peel and scoop out two cups of flesh. Add the butter to the warm flesh and mash smooth with a fork (some fibers and lumps are ok.) Set aside.
Reduce the oven temperature to 350º.
In a medium bowl, whisk together the eggs, sugars, nutmeg and salt until smooth. Whisk in the bourbon, vanilla and milk, then whisk the egg mixture into the sweet potato mixture until combined.
Pour the filling into the warm, par-baked crust (this will reduce the baking time and ensure a crisp bottom crust). Place on a baking sheet to catch any drips, and place the baking sheet in the oven, on the baking stone if using. Bake until the filling is puffed and beginning to crack around the edges and the pie wobbles like jello when shaken (but not in a wet, jiggly way), about 45 minutes.
Pecan Topping:
Spread the pecans on a small baking sheet and toast in the oven until fragrant and crisp, 12 minutes. Let cool until handleable, then use your fingers to break them into small pieces (this results in more regular pieces and less 'dust' than chopping). Combine the pecans in a medium bowl with the sugar, maple syrup and sea salt.
When the pie has finished baking, remove it from the oven and sprinkle the pecan mixture evenly over the top. Sprinkle with the pinches of flaky salt. Return to the oven and bake for 10 or 15 more minutes.
Let the pie cool at least 2 hours, then slice and serve. (For cleaner slices, chill the pie before cutting.)The pie keeps well in the fridge for up to several days, though the crust will be crispest on the day it is baked.
Maple Whipped Cream:
Whip the cream either in a bowl with a whisk or in a stand mixer fitted with the whip, until it holds soft peaks. Add the maple syrup to taste and the vanilla extract.
Notes
Use orange-fleshed sweet potatoes for this pie, such as garnet or jewel.If you lack sourdough starter, use the Flakiest, All-Butter Pie Dough instead.The most efficient order in which to make this pie is as follows:1) Make the pie dough. Chill. (The dough, that is.)2) Roast the sweet taters.3) Roll out the dough, fit it into the pan, chill for 1/2 hour, freeze 20 minutes.4) Par-bake crust.5) While crust bakes, make the pie filling. Pour into the hot crust and6) Bake the pie.7) While the pie bakes, make the pecan topping. 8) Sprinkle topping on pie, bake.9) Cool 2 hours.Sweet Potato Filling: Adapted from Baking Illustrated.Maple Pecan Topping: Adapted from Cook's Country.Nutritional values are based on one of eight servings.