Place the whole hatch chiles under a broiler or directly over a gas flame, turning them every couple of minutes until the skins blister and blacken on all sides. Transfer them to a bowl and cover with plastic wrap to steam for about ten minutes, which makes the skins slide right off. Peel, deseed, and dice the chiles into small pieces, then set them aside while you tackle the pasta and sauce.
Bring a large pot of generously salted water to a boil and add your macaroni or cavatappi. Cook it about a minute less than the package directs so it stays slightly firm, since it will keep cooking in the oven later. Drain the pasta and set it aside, giving it a light toss with a bit of butter so it doesn't stick together while you finish the sauce.
Melt two tablespoons of butter in a medium saucepan over medium heat, then whisk in the flour and cook for about a minute until it smells slightly nutty. Slowly pour in the warmed milk while whisking constantly, and keep stirring until the mixture thickens enough to coat the back of a spoon. This usually takes five to seven minutes of steady whisking, so don't walk away from the stove.
Reduce the heat to low and add the shredded cheddar and Monterey Jack a handful at a time, stirring after each addition until fully melted before adding more. This gradual approach keeps the sauce silky instead of clumpy or greasy. Stir in the garlic powder, salt, and pepper, tasting as you go to make sure the seasoning feels balanced.
Fold the diced hatch chiles into the cheese sauce, then add the cooked pasta and stir everything together until every piece is coated. Pour the mixture into a buttered 9x13-inch baking dish, spreading it out into an even layer so it bakes uniformly. At this point the kitchen should already smell incredible.
Melt the remaining two tablespoons of butter and toss it with the breadcrumbs until evenly coated, then sprinkle the mixture over the top of the pasta. Bake in a preheated 375°F oven for 20 to 25 minutes, until the topping turns golden brown and the sauce is bubbling around the edges. Let it rest for five minutes before serving so the sauce has time to set slightly.