Bring a large pot of salted water to a boil. Add pasta and cook according to package instructions, until al dente. Reserve 1 cup of pasta cooking water, then drain the pasta and return to pot.

Meanwhile, heat a large (12-inch), deep skillet over medium-high. When hot, add 2 tablespoons oil and the mushrooms. Cook for 7 to 8 minutes, giving it a stir every 2 minutes or so.

When the mushrooms start to turn golden, add the garlic, five spice and 1/2 teaspoon each salt and black pepper, then cook for 1 minute. Transfer the mushrooms to a plate and set aside.

While the mushrooms are cooking, make the sauce: In a small bowl, whisk the eel sauce, maple syrup, soy sauce, sesame oil and chili oil (we used Trader Joe's chili onion crunch) with 1/4 cup water until combined. Set aside.

Reheat the skillet back to medium-high. Add the remaining 2 tablespoons oil and the broccolini. Season with salt and pepper and cook for 4 to 5 minutes, tossing occasionally, until the broccolini is tender but still quite crisp.

Add the contents of the pan to the pot of spaghetti, along with the sauce and the butter. Toss to combine.

Add the reserved pasta cooking water, 1/4 cup at a time, to loosen the sauce. When the butter has melted, add the mushrooms and toss again.

Serve topped with scallions and sesame seeds, if desired.