In a small bowl, stir together the warm milk, sugar, and yeast. Set aside for 10-15 minutes or until bubbly.
To a large bowl, stir together the brown rice flour, sorghum flour, tapioca, white rice flour, salt, and xanthan gum to combine. Add the sourdough starter, the bubbly yeast mixture and the softened butter. Stir together to combine then knead with your hands for a few minutes until the dough holds together in a smooth ball.
Cover with a lid or a plate and let rise for 1 hour.
On a gluten free floured board, divide into 8 equal pieces. Roll each into a rope approximately 16" long. Form into a pretzel**.see note below. Move to the parchment lined baking sheet. Repeat with remaining dough pieces. Let rise for 25-30 minutes.
While rising, preheat the oven to 450℉, set a stockpot half full of water (approximately 7- 8 cups of water to boil. When boiling, add in 2 tbsp of baking soda. The water will bubble up so be sure to leave enough space in the pot.
Lift a pretzel with your hands gently or with a spatula into the boiling water. Then repeat with one more. Set a timer for 30 seconds. Turn the pretzel over in the water carefully. Set the timer for 30 seconds more. The pretzel should begin to float. Using a large slotted spoon or spatula, remove to the parchment lined tray. Repeat the process with the remaining pretzels.
Brush with egg wash/water and add your choice of toppings. bake for 16-22 minutes, adding additional egg wash midway through baking if desired.
Remove to a wire rack or cool on the tray. Serve with butter or spicy or regular mustard.