4 tablespoons unsalted butter, divided, plus extra for greasing the pan
2 tablespoons all-purpose flour
1 1/2 cups whole milk
2 ounces (about 1/4 cup) cream cheese
8 ounces (about 2 1/2 cups grated) chihuahua cheese or white cheddar
1 (4 ounce) can chopped green chiles
1 teaspoon kosher salt
1 teaspoon dried ground mustard
1/2 teaspoon onion powder
1 pound short pasta, such as macaroni, orecchiette, or penne
1 1/2 cups fresh breadcrumbs
1/3 cup chopped chives
Heat oven to 375°F and grease a 9 x 13-inch baking dish.
Melt 2 tablespoons of the butter in a medium saucepan over medium heat. Whisk in the flour and continue whisking for 1 minute.
Slowly add the milk, whisking continuously, until all milk has been added. Bring to a simmer and add half the cheese. Stir until melted, then add remaining cheese, chiles, salt, mustard, and onion powder. Stir until all the cheese has melted and mixture is well combined. Remove from heat.
Meanwhile, bring a large pot of heavily salted water to a boil. Add pasta and cook 3-4 minutes less than is instructed on the package. Drain and return to the pot. Add sauce and stir to coat. Transfer pasta mixture to the baking dish.
Place breadcrumbs in a medium bowl. Melt remaining butter and pour over breadcrumbs. Add chives and season with salt and pepper. Stir to combine. Evenly spread breadcrumbs over the pasta and place in the oven.
Bake until cheese is bubbly, about 15 minutes. Turn broiler to high and place under broiler until crumbs are golden and crisp, about 3-4 minutes.