Combine yeast, water, and sugar in a large mixing bowl-mix well . Add butter, egg, salt, and 3 cups flour-mix until smooth continue to mix in. Flour until mixture stiffens. Roll into a ball or log. Wrap in plastic and chill for 4-24 hours. Divide dough into 36 portions. Roll each piece into an 18-inch rope. Form into a pretzel shape, pressing ends to seal. Place onto a parchment-lined baking sheet. Repeat with remaining dough. Combine yolk and water-mix well. Brush over pretzels. Cover and allow to rise for 30-45 minutes. Bake @ 400 degrees for 10-15 minutes. Dip in salt as desired. Serve warm, with mustard to the side.