Cook your noodles in boiling water according to package instructions. Right before draining, scoop out ½ cup of the starchy noodle cooking water and set it aside. Drain the noodles and set them aside.
Melt the butter in a large skillet or wok over medium heat. Add the sliced shiitake mushrooms and cook for 3–4 minutes until they begin to brown. Turn the heat down to medium-low, then add the minced garlic, the white parts of the scallions, and the red pepper flakes. Sauté for 1 to 2 minutes until intensely fragrant, being careful not to burn the garlic.
Add the oyster sauce, soy sauce, 1 tablespoon of the Herbie's Furikake, and ¼ cup of your reserved noodle water directly into the skillet. Whisk or stir vigorously—the starch in the water will marry with the melted butter, creating a smooth, glossy, and bubbling sauce.
Dump the cooked noodles into the skillet. Crank the heat back up to medium and toss everything vigorously for 1 to 2 minutes. As you toss, the noodles will absorb the savory garlic-butter sauce and create a thick coating. (Note: If the noodles look too dry, splash in a little more of the remaining noodle water).
Turn off the heat and stir in the green parts of the scallions. Plate the noodles and aggressively dust the top of each serving with the remaining Furikake for that essential seaweed-sesame crunch.