Bring a couple inches of water to a boil in a medium saucepan, then reduce the heat to maintain a simmer.
Preheat a medium skillet over medium-low heat for the steak. Preheat another medium skillet over medium-low just for 5 minutes before cooking the scallions.
Season the Wagyu with salt and place in the first preheated skillet. Reduce the heat to low and cook until nearly medium-rare, about 10 minutes per side (or adjust for your desired doneness).
While the Wagyu is cooking, make the miso sauce: Whisk the egg yolks with the instant dashi, miso, sake and sugar in a medium metal bowl until smooth. Place the bowl over the simmering water in the saucepan (do not let the bowl touch the water). Whisk occasionally until the eggs have set and the sauce has thickened, about 7 minutes. Remove from the heat but leave the bowl over the saucepan to keep the sauce warm.
Next, collect 1 tablespoon of rendered fat from the Wagyu in the skillet and add it to the second preheated skillet. Add the scallion whites and chiles, season with salt and cook, stirring occasionally, until the scallions start to char, about 2 minutes. Add the scallion greens and cook, stirring occasionally, until lightly charred, 1 minute. Reserve on a plate.
After the Wagyu has cooked for 10 minutes on each side, increase the heat to high and cook until browned and crusty, about 1 minute per side.
Remove the Wagyu from the skillet and slice across the grain. Whisk the miso sauce again. If the sauce is too thick, stir in a bit of hot water from the saucepan to reach your desired consistency. Spoon a small puddle of the sauce onto serving plates, top with the charred scallions and Wagyu and serve.