Bring a large pot of salted water to a boil and cook the bucatini according to package instructions until al dente. Reserve some pasta water before draining.
Heat butter and olive oil in a large skillet over medium heat.
Add the sliced mushrooms and cook until golden brown and caramelized.
Stir in the garlic and cook until fragrant.
In a small bowl, whisk together the miso paste, soy sauce, heavy cream, and a splash of reserved pasta water until smooth.
Pour the miso sauce into the skillet and stir well to combine with the mushrooms.
Add the cooked bucatini to the skillet and toss everything together until the pasta is fully coated in the creamy sauce.
Add parmesan cheese and more pasta water if needed for a silky texture.
Garnish with chopped parsley and freshly cracked black pepper before serving.