• 5 Tbsp (77 g) soy sauce
• 5 Tbsp (77 g) mirin, to taste
• 2 Tbsp, 1 tsp (35 g) sake
• 0.6 piece kombu (dried kelp) (1/3 oz, 10 g per piece; 4 x 4 inches or 10 x 10 cm), dried kelp
• 5 Tbsp to ¹⁄₂ cup, 2 Tbsp katsuobushi (dried bonito flakes), packed

In a medium saucepan, bring sake and mirin to a boil over medium-high heat; cook until the alcohol smell dissipates. Add soy sauce. Add kombu and katsuobushi. Bring to a boil, reduce heat, and simmer 5 minutes. Turn off heat, steep until fully cooled to room temperature, then strain and set aside.