Add bacon to a 5 1/2 quart Dutch oven over medium-heat and cook, stirring, until crispy. Remove bacon with a slotted spoon and set aside.
Add pasta to pot with the rendered bacon fat. Add 4 cups water and one teaspoon salt. Bring to a boil over high heat and cook, stirring frequently, until pasta is just tender and a small amount of water still remains, about 7 minutes.
Add mozzarella, cheddar, fontina, cream cheese, milk, butter, mustard powder, and cayenne. Reduce heat to low, stirring until cheese is melted. Season to taste with salt and pepper. Remove from heat and serve immediately, garnished with reserved bacon, chives, and additional cayenne, if desired.