Preheat the oven to 375°F. Spray a 13 x 9-inch baking dish with nonstick cooking spray.
Boil the macaroni just until al dente. Drain and rinse under cold water.
Melt the butter in a large pot over medium heat. Add the flour, ground mustard, salt, and white pepper, and stir until smooth and combined. Let cook for 2 minutes.
Whisk the milk and chicken broth into the pot, and continue whisking until the sauce is smooth and free of lumps.
Bring the sauce to a simmer, whisking constantly, cooking until it begins to thicken.
Remove the pot from the heat, and stir in 2 ¼ cups of the Cheddar cheese, stirring until melted into the sauce. Add the cooked macaroni to the pot and stir until evenly coated with the sauce.
Pour the macaroni and cheese sauce into the prepared baking dish, and spread to even out. Top with the remaining ¾ cup shredded Cheddar cheese.
Bake for 30 minutes, or until cheese is bubbly and lightly browned. Let cool for at least 5 minutes before slicing. For 100 calorie portions, slice casserole into 36 sections by cutting 4 rows by 9 rows.