Ingredients

• 8 ounces (230 g) macaroni
• 1/3 cup plain flour
• 3/4 teaspoon mustard powder
• 1/2 teaspoon garlic powder
• 1/2 teaspoon kosher salt
• 1/2 teaspoon black pepper
• 1/8 teaspoon cayenne pepper
• 6 tablespoons (85 g) butter (for the sauce)
• 1 1/2 cups (360 ml) whole milk
• 1 cup (240 ml) heavy cream
• 1 pound (455 g) Monterey Jack cheese, shredded
• 1 teaspoon garlic powder (for the butter)
• 16 slices square sandwich bread
• 8 slices mild cheddar cheese
• 8 slices Colby Jack cheese
• 8 tablespoons (110 g) butter (for spreading and griddling)

Equipment

• Medium saucepan or pot
• Colander
• Whisk
• 9-by-13-inch rimmed baking sheet
• Parchment paper
• Large cast-iron or nonstick skillet
• Wide spatula

Instructions

1. Bring a medium pot of well-salted water to a boil (it should taste like the sea) and cook the macaroni, stirring now and then, until al dente, 8 to 10 minutes. You want it tender with a little chew, since it will keep cooking in the sauce.
2. While the pasta boils, whisk together the flour, mustard powder, garlic powder, salt, black pepper, and cayenne in a small bowl so the seasonings are evenly distributed.
3. Drain the pasta and set it aside. Return the empty pot to low heat and melt the 6 tablespoons butter, then whisk in the flour mixture. Cook, whisking often, until it smells nutty and just starts to turn golden, about 1 minute, watching so it does not scorch.
4. Slowly whisk in the milk and cream until smooth, then cook, whisking constantly, until the sauce is hot and beginning to thicken, about 2 minutes. Off the heat, stir in the shredded Monterey Jack a handful at a time until fully melted and glossy, then fold in the drained pasta.
5. Line a 9-by-13-inch rimmed baking sheet with parchment and grease it lightly. Spread the mac and cheese in an even layer, press a second greased sheet of parchment directly onto the surface, and refrigerate until cold and firm, about 1 hour.
6. When ready to cook, heat a large cast-iron or nonstick skillet over medium-low. Stir the remaining butter with 1 teaspoon garlic powder in a small bowl until blended.
7. Peel the top parchment off the chilled mac and cheese and cut it into 8 equal squares, one per sandwich.
8. Spread garlic butter on one side of each bread slice. Lay half the slices buttered side down, top each with a slice of cheddar, a square of mac and cheese, and a slice of Colby Jack, then close with the remaining bread, buttered side up.
9. Griddle as many sandwiches as fit without crowding, cover, and cook until the bottoms are deep golden, about 4 minutes. Flip and cook 4 minutes more, until the second side is browned and the filling is heated through.
10. Repeat with the rest, cut in half if you like, and serve hot while the centers are melty.