Bring the water to a boil in a large pot. Add the cauliflower and cook until soft, around 10 minutes.
In a small pan, cook the garlic in the butter over medium-low for 5 minutes, or until fragrant. Make sure not to burn it, as that will make your sauce bitter.
Drain the cauliflower, reserving 1/2 cup of cooking liquid.
Add the cauliflower, reserved liquid, milk, salt, pepper, and garlic/butter mix to a blender. Blend until smooth.