In a food processor, pulse together all the dry ingredients and the butter into short bursts until the mixture forms pea-sized lumps. Add the water through the feed chute as you pulse until the mixture forms a stiff dough and pulls away from the sides of the food processor bowl. Form the dough into a 6-inch disk and wrap it in plastic; chill for one hour.
Preheat the oven to 375 degrees F. Roll the chilled pie dough into a 12-inch round and press into a 9-inch pie pan. Flute the edges. Bake for 10-15 minutes until the crust is set and beginning to brown slightly. Remove the pie pan from the oven and let it cool. Put the sweet potatoes into a medium pot and cover them with water by an inch. Bring to a boil. Boil slowly until the potatoes are tender with no absolutely no resistance at the center when pieced with a fork. Drain off the water and mash the potatoes with a potato masher. Do not use a potato ricer or food processor. As you mash the potatoes, add the sugar, cinnamon, and nutmeg; then whisk in the eggs, milk, and vanilla. The butter goes in last. Once the filling is well-mixed, pour it into the baked pie crust. Arrange pecan halves around the outside edges and sprinkle the top of the pie with more ground cinnamon. Bake for 20-25 minutes until the filling is set and the edges of the crust have browned. Serve the pie warm or at room temperature with whipped cream!