Meanwhile in a small pot, heat the milk and bring to a simmer.
In a large skillet melt the butter and sauté shallots until soft, then add the flour, stir together and cook for 1 minute over medium heat. Add the milk and bring to a simmer, cook for approximately 10 minutes making sure to stir often, then season with salt and white pepper.
Add 2 cups of the cheddar, the cream cheese and 1/2 jar of pesto; whisk to form a smooth sauce.
Cook the pasta 3 minutes less than the recommended time. Drain and toss the pasta with the sauce.
Spray a 13" x 9" baking dish with non-stick cooking spray. Add 1/2 of the mac and cheese, layer the remaining 2 cups of cheddar and add the rest of the mac and cheese on top.
Bake in a preheated 425°F oven for 15 minutes or until brown and bubbling on top.
Remove from the oven and set aside to rest for 10 minutes. Mix in remaining pesto to taste.
