In a stand mixer, stir together the potato starch, tapioca starch, 100 grams sugar, the sorghum flour, salt, xanthan gum (if using) and orange zest. Add the yeast-psyllium mixture. Snap on the dough hook and mix the dough on low speed. Add the eggs and vanilla. Increase the speed to medium and keep mixing until the dough comes together, 2 to 3 minutes. It will look dry and clumpy for the majority of that mixing time, but by the end, it should come together (or nearly together) into a ball. Add the soft butter, 1 tablespoon at a time, while the mixer is running. Scrape the bowl as needed. Continue mixing for another minute until you have a soft and sticky dough that holds together.
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