Prepare crumb topping by melting butter & letting it cool. Once it's cooled add bread crumbs & shredded cheese. Mix & set aside.
Fill a pot with water for your shells. Heat until boiling, add shells & cook 7-8 min or until directed on the box. Drain shells, saving ½ cup starchy water for later.
In a separate medium/large saucepan, melt butter over medium/high heat. Stir in flour & cayenne pepper, forming a roux. Stir for one minute. Gradually whisk in milk, & stir for 3 minutes or until boiling & thick.
Stir in heavy cream, cheeses, dijon, & spices. Remove from heat & continue stirring until everything is melted nicely.
Add your cooked, drained shells to the cheese mixture, & then add the extra starchy water you saved. Combine well & pour into a large rectangle baking dish that has been buttered or sprayed. Sprinkle topping on & bake at 400˚F for 20-25 minutes until bubbling & golden brown on top.