Preheat the oven to 400°F (200°C), with the convection option on, if you have it. Butter an 8-inch (20-cm) glass or metal baking dish.
Bring a medium saucepan of generously salted water to a boil. Add the pasta and stir immediately. Boil the pasta, stirring occasionally, just until al dente. The timing will vary according to the shape of pasta so you may want to check the package directions for some guidance on time. Drain the pasta in a colander.
While the pasta is cooking, whisk together the flour, mustard powder, garlic powder, black pepper, and cayenne pepper in a small bowl and set aside.
Place the empty pasta pan (no need to wash it!) over medium-low heat and add the butter. When the butter is melted, whisk in the flour mixture. Cook, whisking almost constantly, until the mixture begins to brown and has a pleasant, nutty aroma, about 30 seconds. Watch carefully so it doesn’t scorch or burn.
Slowly whisk the milk and cream into the butter-flour mixture, combining well. Cook, still whisking constantly, until the sauce is heated through and just begins to thicken, about 2 minutes. Remove from the heat.
Add 4 ounces (115 grams) Cheddar and all the Monterey Jack to the sauce gradually, while stirring constantly in one direction with a wooden spoon or rubber spatula. Stir until the cheese is melted into the sauce, then stir in the cooked pasta.
Put the torn bread in a food processor and pulse until fine bread crumbs form, or chop finely by hand. In a small bowl, toss the bread crumbs or panko with the remaining 2 ounces (55 grams) Cheddar.
If you’re baking the chili mac right away, gently rewarm the chili in a small saucepan over medium-low heat until hot. Pour the chili into the prepared dish and spread it evenly. If your chili is on the liquidy end of the spectrum you may want to pour some of it off. Carefully spoon the mac ‘n’ cheese over the chili and sprinkle it evenly with the bread crumb mixture. If you’re making the chili mac ahead of time, pour the cooled chili into the prepared dish and spread evenly. Carefully spoon the mac ‘n’ cheese over the chili and sprinkle it evenly with the bread crumb mixture. Cover the dish tightly with aluminum foil and refrigerate for up to 2 days.
Bake the chili mac, uncovered, until the topping is toasted and crunchy and the sauce is bubbling around the edges, about 20 minutes. (If you're baking the chili mac after it's been refrigerated, you may need to add 10 minutes or so to the baking time.) Let the chili mac cool for about 10 minutes.
Scoop the chili mac into bowls and serve.