In a large mixing bowl, combine warm water and yeast. When the yeast dissolves, add half of the flour, the butter, salt, and sugar and mix for about 3 minutes. Stir in the remaining flour. Turn dough out onto a floured surface and knead until the dough is no longer sticky. Let the dough rise in a greased, covered bowl until it doubles in size, about 45 to 50 minutes. Punch down the dough and divide into 12 pieces. Roll each piece of dough into 18-inch long ropes. On a greased cookie sheet, shape each rope into a pretzel shape. Let rise again until almost doubled, about 30 minutes.
Preheat oven to 500 degrees F.
In a large non-aluminum Dutch oven over high heat, bring 4 cups of water and baking soda to a boil. Using a large spoon, gently lower pretzels into water a few at a time and boil for about 1 minute, or until they float. Return the pretzels to the cookie sheet and brush with melted butter then sprinkle with kosher salt. Bake until browned, about 12 minutes. Serve hot with mustard.