Microwave the cooked udon noodles for 1–2 minutes. Mix together the dashi powder, soy sauce, and mirin. Cut the bacon into thick strips, slice the green onion diagonally, and thinly slice the onion.

Separate the maitake mushrooms into small pieces. Tear the nori sheets into small pieces. (Try to keep the nori dry until use.)

In a frying pan, cook the bacon until the fat starts to render. Add the green onion and onion, and sauté until softened. Add the maitake mushrooms and stir-fry briefly over medium heat.

Add the udon noodles and sake to the pan, and break up the noodles well. Once separated, add the nori and mixed seasonings, and stir-fry over medium-high to high heat until everything is well combined.

Turn off the heat and add the butter. Mix quickly to coat everything evenly. Garnish with chopped green onions or shiso leaves if desired.