Mix 2 tbsp dashi stock, 1 ½ tbsp Japanese soy sauce (koikuchi shoyu), ½ tbsp mirin and ½ tsp light brown sugar in a saucepan and bring to a boil on a medium heat. Allow to boil for 1-2 minutes and then remove from the stove.
Add 2 ice cubes to the sauce to cool it quickly. Once cool, store in the fridge until serving time.
If using wakame, soak according to the instructions on the packaging. (Usually about 5-10 mins.)
Cook the udon noodles according to the instructions on the packaging. Once cooked, pour them into a colander and wash with cold running water. Place a bowl underneath and add a few ice cubes to the noodles. Leave to chill for 5 minutes.
Prepare your toppings according to the instructions in the ingredient list. Divide the udon into serving bowls and arrange the toppings on top.
Add a small amount of wasabi to the side of the bowl (optional) and drizzle with sesame oil. Finally, pour the chilled sauce over the top.