Cook the spaghetti according to the package directions, reserving ¾ cup of the cooking water. Drain the pasta and return it to the pot.
Meanwhile, heat 2 tablespoons of the butter in a large skillet over medium heat. Add the Brussels sprouts and cook, tossing occasionally, until crisp-tender, 4 to 6 minutes.
Add the garlic and cook until fragrant, 1 minute.
Add the Brussels sprouts, cream, the remaining 2 tablespoons butter, ½ teaspoon salt, ¼ teaspoon pepper, and ½ cup of the reserved cooking water to the pasta and toss to coat. (Add more cooking water as needed to loosen the sauce.)