Cook ingredients - Add kombu, katsuobushi, mirin, and rice vinegar in a small saucepan and cook on low until the mixture just simmers. Give it a stir, remove from heat, and allow the mixture to cool. The mixture will smell sweet and smoky, and the Kombu will soften.
Strain - Once the mixture cools after about 10 minutes, strain it to remove the katsuobushi and kombu and pour the mixture into a jar.
Shake - Add the soy sauce and yuzu juice to the jar. Secure the lid on the jar and give the mixture a good shake until combined.
