For the sauce, simply mix everything together until well-incorporated. Feel free to adjust to your desired taste.
Slice or break apart your mushrooms.
Heat your pan. I used 9-inch cast iron pan. Over medium heat, melt the vegan butter.
Add in the mushrooms and leave to cook over medium to medium high heat for 3-4 minutes until slightly browned on one side. Flip or mix to brown the rest and allow for any excess moisture to draw out from the mushrooms.
Lower the heat to medium and add the mirin, sake, soy sauce, and sugar. Mix well. This will start to thicken from the sugars and cast the mushrooms.
Afterwards, move the mushrooms to the sides of the pan to create a round opening in the centre to place the rice on.
I simply packed my rice in a large bowl and flipped it over to create a dome shape.
Add lots of freshly ground pepper over the rice and mushrooms. Add the corn kernels, more pepper (if desired), chopped scallions, and then pour in the miso mayo sauce.
Mix everything together and enjoy while hot.