Mix together flour blend, potato flour, yeast, xanthan gum and salt.
Using the paddle attachment of your mixer, beat ingredients together on medium speed, adding melted butter and 11/4 cup warm water. Add enough water to make a soft but not wet dough. Beat for 5 minutes. Then cover and let sit for 5 minutes.
Uncover and beat an additional 4 minutes. Oil dough lightly. Cover bowl with plastic wrap and place it in a warm place for 30 minutes.
Divide dough into 12 pieces. Roll each into a 12-inch rope. Make a U-shape with the rope. Holding the ends, cross them over each other and press them onto the rope to form a pretzel. Transfer to a parchment-lined pan. Cover with lightly oiled plastic wrap and let rise for 20 to 30 minutes.
Preheat oven to 450 degrees. Line 2 half-sheet pans with parchment paper and lightly brush with vegetable oil. Set aside.
Bring baking soda and 10 cups water to a boil in a large saucepan. Lower heat to simmer and cover.
Place pretzels into boiling water, one at a time, and boil for 30 seconds. Remove from water with a slotted spoon.
Return boiled pretzels to the sheet pan. Brush each with egg yolk and water mixture. Sprinkle with salt.
Place in preheated oven and bake until dark golden brown, about 12 to 14 minutes. Transfer to a rack and cool for at least 5 minutes before serving.
TIP For an authentic look and taste, sprinkle pretzels with coarse sea salt or pretzel salt.