To make the crust: In a medium-sized mixing bowl, combine the flour, buttermilk powder, salt and baking powder. Using a pastry blender, your fingers, or a mixer, cut in the butter and vegetable shortening, leaving some small pea-sized lumps., Mix the vinegar with 3 tablespoons (43g) of the water. Sprinkle this mixture over the flour and fat., Squeeze the dough together; if it's not cohesive, add an additional 1 to 2 tablespoons water (just enough to make the dough stick together)., Shape the dough into a flattened disk, wrap it in plastic wrap, and refrigerate it for 30 minutes or longer., Preheat the oven to 425°F., Remove the dough from the fridge. If it's been refrigerated longer than 30 minutes, allow it to rest for 10 to 15 minutes, until it's pliable., Roll out the dough to about 14" in diameter; it'll be about 1/8" thick., Carefully transfer the dough to a 9" pie pan (a giant spatula works well here), fold the edges under, and crimp. (See "tips," below for details on choosing your pie pan.) You want to form a high edge to hold the streusel and filling, so do a "stand-up" crimp rather than one you simply flatten with the tines of a fork., Prick the bottom of the crust with a fork or dough docker, and line it with pie weights (or a perforated nesting pie pan) to help the shell hold its shape., Bake the crust for 15 to 18 minutes, until it's lightly browned; this is called "prebaking.", Remove the crust from the oven, and reduce the oven heat to 400°F., Allow the crust to cool slightly before removing the weights or nested pan. If the crust still managed to bubble, just prick it gently with a fork; it'll deflate as it cools., To make the peach and custard filling: In a medium-sized mixing bowl, whisk together the sugar, flour and salt., Add the eggs, cream, rum, and vanilla, and whisk together well., Let the mixture sit for 5 minutes, or the time it takes to peel and dice the peaches. (This resting time allows the sugar to dissolve.), Whisk the custard again and pour it into the pre-baked pie shell, then spoon the peaches on top. Some will sink; that's OK, they're supposed to., Use a pie crust shield or aluminum foil to gently cover the outer edge of the crust., Bake the pie for 20 minutes. Make the topping while the pie bakes., To make the topping: If you have a food processor, place all the topping ingredients into the work bowl and process, using the steel blade, until crumbs begin to form. To make by hand, whisk together the minced crystallized ginger, sugar, and flour, then stir in the melted butter., After the pie has baked for 20 minutes (it will have just begun to set up around the edges), carefully sprinkle the streusel over the top, leaving the pie crust shield in place., Bake for another 20 to 25 minutes, or until the pie has puffed, the streusel is very slightly browned at the edges, and/or a digital thermometer inserted into the center of the pie reads about 160°F., Remove the pie from the oven, and cool it on a rack for about 2 hours before serving., Storage information: Refrigerate any leftover pie for up to 4 days.