Break cauliflower into small pieces
Add the cauliflower to a large saucepan with 2 T butter, ½ t salt, and water over medium/high heat and cover with a lid
Cook cauliflower for about 5 minutes, until mostly tender
Remove lid and let cauliflower sauté for a minute or two, or until lightly caramelized
Turn heat off, cover, and keep warm until ready to toss with the cheese sauce
Heat remaining 2 tbsp butter in a saucepan over medium / high heat
Add squash, carrot, onion, garlic powder, mustard, and remaining salt to saucepan
Sauté 5 minutes, or until onion is translucent
Add coconut milk to the saucepan and bring to a simmer
Allow to simmer for about 10 minutes, or until vegetables are tender and coconut milk has reduced by ⅔ and is thick
Pour veggies and coconut milk into blender/food processor and puree until smooth
Add egg yolk to puree and blend well
Check seasonings and add more salt or pepper to taste
Pour sauce over hot cauliflower and toss to coat