Mix mentaiko, butter, japanese mayonnaise, and dashi concentrate in a bowl. Set aside. (For mentaiko, use your finger or spoon to squeeze and scrape out the roe from the sack)
Bring a pot of water to a boil and cook the noodles for 1 minute 30 seconds. Once the noodles are cooked, drain well and transfer the noodles to the bowl from step1. Toss the noodles with the mentaiko sauce.
Place the noodles on a plate, garnish with chives and nori. Enjoy!