With food processor, grind pistachios and until fine. Combine with plain breadcrumbs and pepper; set aside.
In a pot, combine milk, butter, yogurt, mozzarella cheese, and 2 1/2 cups of cheddar. heat on low to medium heat. Stir often until heated through and smooth.
While mixture heats, cook paste al dente; drain. Place pasta into casserole dish (sprayed with cooking spray).
Add bacon to milk/cheese mixture and pour over pasta. Top with the bread crumb mixture and remaining cheddar.