In a food processor, combine the sweet rice, oat, and teff flours with the cornstarch, tapioca flour, ground chia seed, sugar, and salt. Place the sliced butter over the top, put the lid on the processor, and open the pouring chute, but don’t pulse yet. In a small pitcher, combine the ice water and buttermilk. Begin pulsing the processor, simultaneously pouring in the liquid in a slow, steady stream. Remove the lid and give the dough a squeeze; it should feel moist but not sticky, and hold together, with lots of pea-size butter pieces. If it feels dry, sprinkle in a teaspoon or two more ice water and pulse quickly to combine until the dough is moistened enough to hold together when squeezed. Gather the dough into a ball, place on a sheet of plastic wrap or beeswax wrap, flatten into a disk, wrap tightly, and chill until firm, 30 to 60 minutes or up to 1 day.