Put the yellow miso, hatcho miso, egg yolks, sugar and sake into a blender or small food processor and run until smooth
Add the oil to a frying pan over medium heat and saute the shiitake, scallion, and ginger until it's well browned and starting to caramelize.
Add the pork and stir-fry, using a spatula to break up the meat into small crumbs.
When the meat is mostly cooked, add the miso sauce and turn down the heat to low. Cook, stirring constantly until the miso is thick enough so that when you run a spatula across the pan, the miso doesn't flow back into the gap created.
When the niku miso is done, stir in the toasted sesame seeds.
This recipe makes about 1 1/2 cups of niku miso.
In the US, yellow miso is often mislabeled "white miso". True white miso (shiromiso) is a unique style of miso from Kyoto that's made with a larger percentage of rice, which makes the miso very sweet. If you bought something labeled "white miso" in the US, chances are you have regular yellow miso. If you're unsure what kind of miso you have, taste a small amount, if it's very sweet, reduce the amount of sugar in this recipe.