Butter an 8-inch (20cm) square baking dish or another shallow baking dish with a 6 cup (1,5L) capacity.
Melt the butter over low heat in a large heavy-duty saucepan. Add the flour and stir constantly for two minutes. Slowly add the milk, starting with about 1/4 cup (60ml) at a time, stirring constantly, until all the milk is added. Increase the heat to medium and bring to a low boil, continuing to stir, then cook for 2 to 3 minutes stirring constantly until thickened. (If it appears lumpy you can give it a few strong stirs with a whisk while the sauce is cooking.)
Remove from heat and stir in the salt, 8 ounces (about 2 1/2 cups/225g) of the mixed cheese, the chipotle powder and some black pepper. Stir until the cheese is melted.
Preheat the oven to 350ºF (180ºC). Bring a large saucepan of lightly salted water to a boil. Cook the pasta until it's very al dente, about a minute less than the recommended cooking time. Drain, rinse in cold water, and drain well.
Stir the cooked pasta into the saucepan of cheese sauce then spread the mixture in the prepared baking dish. Sprinkle the top with the remaining 2 ounces (1/2 cup, 60g) of the grated cheese mixture then top with the Parmesan. If desired, sprinkle a little chipotle powder over the top.
Bake for 20 minutes until the mac & cheese is bubbling. If you want the top darker, turn on the broiler and watch carefully, removing it when it gets to the desired brownness. (Don't overdo it, though.) Remove from the oven, let cool 5 minutes, then serve.