2) Mix the sweet potatoes and butter in the food processor until smooth. Add the remaining ingredients and pulse to combine. Set aside.
For the pastry case:
1) Pulse the flour, sugar and salt in a food processor with the metal blade until combined. Add the fat and pulse about 10 times. Add the butter and continue to pulse until it resembles cornmeal mixed with bean-size bits of butter.
2) Beat the egg yolk and vinegar together, add to the processor and pulse 3 to 4 times, but don't let the dough form a ball in the machine. Remove the blade, and gather the dough together by hand. If dough does not come together, sprinkle up to 1 tbsp of ice cold water over it and bring together.
3) Wrap the dough in cling film and shape into disk. Refrigerate at least 1 hour or up to 2 days.
4) Lightly dust the counter with flour. Roll the dough into a 30cm circle and transfer to a prepared pie tin. Trim the dough so that the edges hang about 1cm over the tin; fold the edges under and flute as desired.
5) Pierce the crust all over with a fork, and refrigerate for at least 20 minutes or up to a day. Position a rack in the lower third of the oven and heat the oven to 220C/Gas mark 7.
6) Line the case with foil or baking paper, add pie weights or dried beans and place on a baking tray. Bake until the crust sets and begins to brown around the edges, about 25 minutes.
7) Lift the foil and weights out of the crust, lower the oven temperature to 190C/Gas 5 and continue to bake until the crust begins to brown on the bottom, about 10 to 12 minutes more.
8) Pour the filling into the warm crust and bake until set, about 20 minutes. Remove from the oven and cool on rack
For the topping:
When ready to serve, whip the cream with the whiskey until it holds soft peaks. Top the pie with whipped cream and candied pecans. Serve.
For the candied pecans:
1) Preheat the oven to 180C/Gas 4. Lightly beat the egg white until frothy. Whisk in the sugar and cinnamon, and then toss pecans in the mixture.
2) Evenly spread the nuts on a baking paper lined baking tray. Bake until dry and toasted, about 10 to 15 minutes. Cool.
Busy Bakers Tips:
The crust can be made, rolled and transferred to the pan, then tightly wrapped and frozen for up to a month. The candied pecans can be made and stored in an airtight container for up to 2 weeks.