Make psyllium gel by combining psyllium husk powder with 320 g of the water in a small bowl; a gel should form after 30-45 seconds
In the bowl of stand mixer, combine the instant yeast, light brown sugar, tapioca starch, millet flour, sorghum flour, xanthan gum, and salt
Make a well in the middle of the ingredients and add the psyllium gel and remaining 320 g warm water
Mix the dough on medium until well combined. It'll be soft and sticky to the touch - that's fine
Preheat the oven to 450F while you start shaping the pretzels
Turn out the dough on a lightly floured surface. Divide into 12 equal pieces (about 112 g each)
Roll each piece of dough out into a 16-18 inch rope, then gently shape into a pretzel shape and put on a parchment - lined baking sheet
Make the baking soda bath by combining the 80 g of baking soda with 5 cups of water (or use the same ratio in an amount that makes sense for the pot you're using). Bring to a slow boil - be careful, because it can boil over!
Working in batches, put each pretzel in the bath for 20-30 seconds (can do more than one at a time if your pot is large enough). Put back on the baking sheet after soak
Brush soaked pretzels with egg wash and sprinkle with pretzel salt or sea salt
Bake in the oven for 12-14 minutes, until golden brown
Brush with optional butter immediately after removing from oven