Cut the stalks and leaves off of the cauliflower and chop into small pieces. Add cauliflower to a large sauté pan with 2 tbs of butter, 1/2 tsp of salt, and water.
Place on medium/high heat, and cover with a lid. Let the cauliflower cook inside the pan for about 5 minutes, until mostly tender. Check the water levels every few minutes to make sure that it doesn’t all evaporate.
When almost tender, remove the lid, and let the cauliflower sauté for a minute or two, or until lightly caramelized in spots. Turn heat off, cover, and keep warm until ready to toss with the cheese sauce.
Cut squash into small cubes. Dice up the carrots and onion.
Heat 2 tbs of butter in a saucepan over medium/high heat. Add the squash, carrot, onion, garlic powder, mustard, and 1 tsp of salt to pan.
Sauté for 5 minutes, or until onion is translucent. Add the can of coconut milk to the saucepan and bring to a simmer. Allow to simmer for about 10 minutes, or until the vegetables are tender and the coconut milk has reduced by 2/3 and is thick.
Pour the vegetables and coconut milk into a blender and puree until very, very smooth. If using, immediately add an egg yolk to the puree and blend well. The hot puree will cook the egg which will thicken the sauce and give it a rich texture.
Check seasonings and add more salt or pepper to taste. Add nutritional yeast. Pour over the hot cauliflower and toss to coat. Top with nutritional yeast for more of a cheesy flavor.