Ingredients

• 8 ounces macaroni
• 1/3 cup all-purpose 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 butter (for the sauce)
• 1 1/2 cups whole milk
• 1 cup heavy cream
• 1 pound Monterey Jack cheese, shredded
• 4 tablespoons butter, softened (for garlic butter)
• 1 teaspoon garlic powder (for garlic butter)
• 16 slices square sandwich bread
• 8 slices mild cheddar cheese
• 8 slices Colby Jack cheese
• 4 tablespoons butter, for the skillet

Equipment

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

Instructions

1. Bring a medium saucepan of well salted water to a boil (it should taste like seawater). Add the macaroni and cook, stirring now and then, until al dente, 8 to 10 minutes. You want it tender but still with a little chew, since it will soften more in the sauce.
2. While the pasta cooks, whisk together the flour, mustard powder, 1/2 teaspoon garlic powder, salt, black pepper, and cayenne in a small bowl so the seasonings distribute evenly.
3. Drain the pasta. Set the empty pot over low heat and add the 6 tablespoons butter. Once it melts, whisk in the flour mixture and cook, whisking often, until it smells nutty and just starts to turn golden, about 1 minute. Watch it closely so it does not scorch.
4. Slowly whisk in the milk and cream until smooth. Cook, whisking constantly, until the sauce is hot and begins to thicken, about 2 minutes. Off the heat, stir in the Monterey Jack a handful at a time until it melts into a smooth sauce, then fold in the drained pasta.
5. Line a 9-by-13-inch rimmed baking sheet with parchment and grease it lightly with butter or cooking spray. Spread the mac and cheese evenly into the pan. Press a second greased sheet of parchment directly onto the surface, then refrigerate until cold and firm, about 1 hour, so it slices cleanly.
6. When ready to build, heat a large cast iron or nonstick skillet over medium-low heat so the bread browns before it burns.
7. Stir the 4 tablespoons softened butter with 1 teaspoon garlic powder until blended into a smooth garlic butter.
8. Peel the top parchment off the chilled mac and cheese and cut it into 8 equal squares. Each square fills one sandwich.
9. Spread garlic butter on one side of each bread slice. Set 8 slices buttered side down on a cutting board. Top each with a slice of cheddar, a square of mac and cheese, then a slice of Colby Jack. Close with the remaining bread, buttered side up.
10. Place as many sandwiches in the skillet as fit without crowding. Cover and cook until the bottoms are deep golden, about 4 minutes. Flip and cook until the second side is browned, the cheese is melted, and the center is hot, about 4 minutes more.
11. Repeat with the rest. Cut in half if you like and serve right away while gooey.