Preheat the oven to 175℃ (350℉). You will need a 23 cm (9 inch) pie dish.

To make the pastry, cut the butter into 1 cm (1/2 inch) pieces and return it to the refrigerator while you prepare the flour mixture. Place the flour, sugar and salt in the bowl of a food processor fitted with a steel blade and pulse a few times to mix. Add the butter and shortening. Pulse 8 to 12 times, until the butter is in pea-sized bits. Pour the iced water down the feed tube and pulse the machine until the dough begins to form a ball. Tip out onto a floured board and roll into a ball. Cover in plastic wrap and refrigerate for 30 minutes.

Roll out on a well-floured board into a circle, rolling from the center to the edge, turning and flouring the dough to make sure it doesn’t stick to the board. Fold the dough in half, place in a pie pan, and unfold to fit the pan.

To make the filling, boil the sweet potato with water in a medium saucepan for 40 to 50 minutes, or until soft. Drain and rinse under cold water, remove the skin and break it up in into pieces. In an electric mixer, beat the sweet potato and the butter. Add the sugar, milk, eggs, nutmeg, cinnamon and vanilla. Beat on a medium speed until the mixture is smooth. Pour the filling into the prepared pastry base/pie crust.

Bake for 55 to 60 minutes, or until a knife inserted in the centre comes out clean. The pie will puff up as it cooks and sink as it cools. Serve warm or at room temperature. Serves 8 - 10.