Preheat oven to 350°F.

Transfer DORITOS^® COOL RANCH^® Flavored Tortilla Chips to large sealable bag; using meat mallet, finely crush.

Cook shells pasta according to package directions and drain. Transfer to greased 13 x 9-inch baking dish.

Meanwhile, in large cast-iron skillet set over medium heat, melt butter. Sprinkle in flour and mustard powder. Cook, stirring, for 2 to 3 minutes or until smooth. Slowly whisk in milk and cream. Whisking, bring to a boil until starting to thicken. Season with salt and pepper. Reduce heat to low.

In batches, stir in cheddar and Gruyère until melted. Pour sauce over pasta. Stir to combine. Sprinkle top with crushed DORITOS^®.

Bake for 25 to 30 minutes or until golden brown and bubbly. Let stand for 10 minutes before serving.