Udon Noodle Soup Kake Udon

Name: Udon Noodle Soup (Kake Udon)
Ingredients: 2 Tbsp. Diamond Crystal or 1 Tbsp. plus ½ tsp. Morton kosher salt; 3¼ cups (406 g) all-purpose flour; Potato starch or cornstarch (for dusting); 2 dashi stock powder packets (preferably Kayanoya); 3 Tbsp. soy sauce; 2 Tbsp. mirin (sweet Japanese rice wine); 2 tsp. sugar; ¼ tsp. kosher salt, plus more; 4 scallions, thinly sliced; Shichimi togarashi (for serving)