Cook the pasta according to package directions. Drain and set aside.
Melt butter in a large saucepan over medium-low heat. Add flour and stir constantly just until roux puffs slightly, about 1-2 minutes. Stir in the broth, milk, dry mustard and salt until smooth.
Increase heat to medium-high. Add cauliflower and bring milk-broth to a rapid simmer, stirring frequently. Immediately reduce the heat back to medium-low, cover and simmer until cauliflower is very tender, about 10-12 minutes, stirring occasionally.
Carefully transfer cauliflower and milk-broth to a blender. Firmly hold the lid to prevent hot liquid from escaping and puree until completely smooth, about 20-30 seconds. Return puree to pan over medium-low heat. Add the shredded cheese and cooked macaroni, stirring well until cheese has melted. Serve immediately with fresh parsley and additional salt and pepper to taste, if desired.