Preheat oven to 375 degrees Fahrenheit.
Cut butter (or preferred fat) into 1 Tablespoon sections. Place in freezer for 10 minutes.
Put cassava flour, arrowroot and tapioca into food processor (or work bowl, if you're using handheld pastry blender). Add sea salt. Blend to mix. Remove cold butter from freezer and cut tablespoons in half, so they are in 1/2-tablespoon rectangles/sections. Add these to food processor and pulse to blend, until largest pieces are pea size (or a bit smaller if using a pastry blender by hand).
Add egg, water and apple cider vinegar, and pour over flour in a donut-shaped gully. Pulse. Use Pulse button until mixture begins to come together: see photo — about 10 seconds.
Dump out contents of food processor onto a large square of parchment paper. (Reserve half if you've doubled the recipe to cut out shapes for the top.) Form into a circle with your hands, pressing and patting it together. The circle will be about 7" in diameter. Place second large square of parchment paper onto dough and begin rolling out with rolling pin, from the center, until you have an 11"-12" circle. (Ideally the center is rolled thinly, as this creates a flakier texture.)
Transfer pie crust to pie plate by peeling off one side of parchment, placing pie plate upside down on pastry, sliding your hand under pastry that still has parchment, then flipping the whole thing over using both hands. If any breaks off you'll be able to piece it back together. Carefully peel remaining parchment paper off of top.
Allow crust to slide down into pie plate, fitting into concave shape. Fold over and flute/crimp top edges. If dough breaks during transfer, it easily pieces back together. Dust your fingers with cassava flour if edges are sticky. To bake the crust blind (without filling), prick all over with fork and bake in preheated oven for about 30 minutes. (To prevent the sides from collapsing in a couple of places, you can line the crust with parchment and pie weights for the first 10 to 15 minutes of baking; then empty and continue for remainder of baking time.) Otherwise fill and bake according to chosen recipe. If you wish, brush crimped edges of pie with egg yolk, to create a really golden outcome.