1) For the crust: Whisk together the flours, buttermilk powder (if using), baking powder, and salt in a medium bowl. Cut the butter into small cubes, and work it into the dry ingredients using your fingers, a pastry blender or fork, or a mixer until the mixture is unevenly crumbly. Sprinkle in the orange juice a tablespoon at a time, continuing to mix until the dough is cohesive. Grab a handful; if it holds together when you squeeze it gently, you've added enough liquid.
2) Pat the dough into a 6" diameter disk on a piece of plastic wrap. Make sure the edges are smooth. Wrap and refrigerate it for at least an hour, or for as long as 3 days.
3) For the filling: Cook the sweet potatoes by boiling, microwaving (drizzle with the cranberry or apple juice and cover), or baking them. Mash the cooked potatoes: you should have about 1 1/4 cups after cooking.
4) When you're ready to bake, roll out the crust to a 12" circle. Transfer it to the prepared pan, and crimp the edges. Refrigerate while you make the filling.
5) Mix the mashed sweet potatoes with the eggs, sugar, half & half (or cream), vanilla, cinnamon, salt, and allspice. For a smooth pie, purée the mixture in a food processor. If you'd rather have more texture, leave everything chunky.
6) Chop the dried pineapple and crystallized ginger into fine pieces (you can pulse in a food processor if you like).
7) To assemble the pie: Preheat the oven to 425°F. Remove the pastry-lined pie pan from the refrigerator and sprinkle the diced pineapple and ginger into it. Pour the filling over, and bake for 15 minutes. Reduce the oven temperature to 350°F, and place foil or a pie shield on the edge of the pie to keep it from getting too dark.
8) Remove the pie from the oven, take off the shield or foil, and cover the top with the mini marshmallows. Return the pie to the oven and bake until a knife inserted 1" from the edge comes out clean, about 20 minutes more.
9) To make the garnish: Place the pecan halves, sugar and salt in a large frying pan. Cook the mixture over medium heat, stirring frequently, until the sugar melts and caramelizes on the nuts; watch carefully as the sugar starts to melt, and stir constantly at that point. It can burn easily if you're not paying attention.
10) Once the sugar melts and turns golden brown, remove the pan from the heat, and continue to stir until the nuts are coated. Transfer the nuts to a piece of parchment paper or foil to cool. Arrange the nuts on top of the pie, which is best served 30 minutes to 1 hour after baking. The top will hold itself up a little longer if you staled the marshmallows in advance (see tip, above left).