When it comes to a boil let it simmer a bit to evaporate the alcohol.
Don't let it boil for too long. If you use mirin seasoning instead of real mirin, you can skip the boiling step.
Add the soy sauce and simmer for 20 to 30 seconds (until it's bubbling around the edges of the liquid).
Add the dashi stock granules, dissolve and it's done!
Let it cool down, then transfer to a clean storage container. Store in the refrigerator. To use, dilute by 5 x to use as a noodle soup where the noodles are in the soup; 3 x to use as a noodle dipping sauce. Adjust to taste.
If you store it at Step 4 (before adding the dashi stock granules) it's the 'kaeshi' that's made by soba restaurants. This can be diluted with dashi stock made from bonito flakes and konbu seaweed to make an authentic noodle sauce.
Use 1/2 teaspoon of the mentsuyu concentrate to 1 egg to make dashimaki tamago (rolled omelette)! This is the only flavoring you need.
Story Behind this Recipe
This is a method I saw a long long time ago on TV. However, I didn't want to bother making the dashi from scratch so I adapted the recipe to use dashi stock granules. It's easy and tasty, and can also be kept for a long time.
Don't let the sauce boil or simmer for too long, or it may get burned. You can use it right away for cold noodles by adding ice cubes to it, but it tastes much better if you let it rest in the refrigerator for a few days I think, since the soy sauce becomes milder in flavor.