Spicy Mac and Cheese Soup

2 (7.25 ounce) packages macaroni and cheese mix

¼ cup butter

2 cups chicken broth

1 (14.5 ounce) can diced tomatoes with green chile peppers

1 tablespoon garlic powder

1 teaspoon ground cumin

1 teaspoon dry mustard powder

½ teaspoon ground black pepper, or to taste

1 ½ cups milk

¾ cup sour cream, divided

Bring a large pot of lightly salted water to a boil and stir in macaroni from the packages, retaining cheese mix packets for later use. Cook macaroni at a rolling boil over medium heat, stirring occasionally, until tender but still slightly firm, 8 to 10 minutes. Drain and rinse macaroni with cold water. Return cooked macaroni to the pot.

Mix butter, chicken broth, and tomatoes with macaroni and bring mixture to a simmer over medium heat; stir in garlic powder, cumin, dry mustard, pepper, and contents of cheese mix packets. Stir in milk and reduce heat to low; heat soup almost to a simmer. Do not boil.

Serve soup in bowls and top each serving with about 2 tablespoons of sour cream.