Butter 13x 9-inch baking dish or individual gratin dishes. Preheat oven to 350°F.
In large, heavy skillet add butter and shallots. Sauté over medium-low heat for 2 minutes, or until translucent. Whisk in flour and cook 1 minute. Do not let flour and butter brown.
Gradually whisk in milk and cream. Stir to break up any lumps. Simmer until mixture thickens slightly, about 3 minutes.
Reduce heat to low and add cheddar, 1 cup at a time, mixing to incorporate. Add 1 cup blue cheese. Whisk until cheese is melted, about 2 minutes. Taste sauce and add salt and pepper to taste.
Fold in cooked pasta and stir to coat. Transfer mixture to baking dish. Sprinkle with panko, remaining blue cheese and sliced prosciutto.
Bake 20-25 minutes or until bubbly. Allow to rest 5 minutes before serving.