Cook the pasta according to the package directions. Reserve 1 cup of the pasta cooking water before draining.
Meanwhile, heat 1 teaspoon of olive oil in a large pot over medium heat. Add the garlic and sauté until fragrant, about 30 seconds. Transfer to a high-powered blender.
Using the same pot, bring about 4 cups of water to a boil. Add the cauliflower florets, cover with a lid, and cook until the cauliflower is fork-tender, 8 to 10 minutes.
Drain the cauliflower and place it in the blender. Add the reserved pasta water, the remaining 2 teaspoons of oil, butter, milk, hemp hearts, lemon juice, Parmesan, salt, and pepper; blend until smooth and creamy. Season with salt and pepper to taste.
Transfer the sauce along with the pasta back to the pot, toss, and heat over medium-low until heated through, 1 to 2 minutes.
Garnish with fresh parsley and additional parmesan, if desired.