Make your ice water by filling a small cup with ice and topping it with cool water. Set aside while you get on with the recipe.
In a large bowl, combine the flours, cornstarch, sugar, xanthan gum, and salt, and stir to combine. Scatter the butter slices over the top, and work with a pastry blender until the butter it in almond-sized chunks. Tablespoon by tablespoon, dribble in the ice water, tossing with a rubber spatula and/or your hands after each spoonful, until the dough is just damp enough to hold together when you give it a squeeze. Add drops of water directly to the floury bits if needed; you may need more water, or less, depending on how warm and worked-in your butter is. When finished, the dough should look like a pile of fat crumbles. We'll bring it together in the next step.
Dump the dough out onto the counter, and quickly fraisage it by scraping an eighth of the dough at a time across the counter beneath your palm. Scrape it up with a bench scraper, and gather it into a ball. Place on a piece of plastic wrap, flatten it into a 6" diameter disc, wrap, and chill for at least 30 minutes or up to 2 days. (The dough can be slipped into a freezer bag and frozen for up to a month or two. Defrost in the refrigerator before proceeding.)
When you're ready to use the dough, let it soften at room temperature for 5-10 minutes. The dough is a touch more delicate than regular pie dough, so it benefits from slightly softer butter. Gently roll it out on a surface dusted lightly with millet flour, dusting the pin, your hands, the dough, and the work surface with just enough flour to keep it from sticking. Proceed with your recipe from here. If the dough tears, pinch it back together, or use a scrap of dough to patch it up.
This is the flour combination that worked for me. Others might work, too (for instance, sorghum flour instead of oat), but I would keep the sweet rice flour, such as Mochiko brand (regular rice flour probably won't work the same way), cornstarch and xanthan gum as they mimic the glutens in wheat flour. I've also made a version with buckwheat flour and have posted that variation below.I like using the fraisage method (scraping portions of the dough across the counter with your palm) to bring the dough together, but if that freaks you out, you can just give the dough a quick knead in the bowl until it sticks together in a ball.This dough is a bit more delicate to roll out than a wheat-based pie dough, so be gentle with it, use plenty of flour, and keep a metal bench scraper or spatula nearby to help you turn and flip the dough as you roll it out. I like to get it started by using a pressing motion with the rolling pin. If it cracks or tears, smush it back together with your hands, or use dough scraps to repair tears in the final dough.The dough is pictured here made into an Apricot Mascarpone Galette, and Apple Quince Tart with Buckwheat Crust.All ounce measurements are by weight.Nutritional values are based on one of ten servings.