Preheat the broiler. Bring the cauliflower and milk to a boil in a large saucepan. Reduce the heat to medium, cover and simmer, stirring occasionally, until tender, about 8 minutes. Transfer to a blender and puree until smooth; return to the saucepan and stir in 1 1/4 cups cheddar and the parmesan.
Meanwhile, grate the tomato into a bowl and stir in the barbecue sauce. Season the chicken with 1/4 teaspoon salt; set on a baking sheet and broil until golden, about 7 minutes. Flip the chicken, brush with the barbecue sauce mixture and continue broiling, basting with the pan juices occasionally, until cooked through, 6 to 7 more minutes.
Meanwhile, bring a medium saucepan of lightly salted water to a boil. Add the pasta and cook as the label directs; reserve 1/2 cup cooking water, then drain. Add the pasta to the cauliflower mixture along with a splash of the reserved cooking water. Cook over low heat, stirring, until the pasta is coated. Top the mac and cheese with the remaining 1/4 cup cheddar; serve with the chicken.
Photograph by Justin Walker
Courtesy of Food Network Magazine