Zaru Soba (Cold Soba Noodles)

Name: Zaru Soba (Cold Soba Noodles)
Prep time: 5 min
Cook time: 10 min
Total time: 15 min
Cuisine: Japanese
Ingredients: ¼ cup sake; ½ cup soy sauce; ½ cup mirin ((plus 1 Tbsp, to taste)); 1 piece kombu (dried kelp) ((1 x 1 inch, 2.5 x 2.5 cm per piece)); ½–1 cup katsuobushi (dried bonito flakes) ((packed; if you like a more intense flavor, use the larger amount)); 14 oz dried soba noodles (buckwheat noodles) ((3.5 oz, 100 g per serving)); shredded nori seaweed (kizami nori); 6 Tbsp mentsuyu (concentrated noodle soup base); 1⅛ cups iced water; 2 green onions/scallions; wasabi ((optional))