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 grilling

Equipment

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

Instructions

1. Bring a medium saucepan of heavily salted water to a boil (it should taste like the sea). Add the macaroni and cook, stirring now and then, until al dente, 8 to 10 minutes. You want it tender with a little chew, since it will soften more in the sauce.
2. While the pasta cooks, 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. Set the empty pot (no need to wash it) over low heat and melt the 6 tablespoons butter. Whisk in the flour mixture and cook, whisking often, until it smells nutty and just starts to brown, about 1 minute. Keep an eye on it so it does not scorch.
4. Slowly whisk in the milk and cream until smooth. Cook, whisking constantly, until the sauce is hot and just begins to thicken, about 2 minutes. Pull it off the heat, then stir in the Monterey Jack a handful at a time until fully melted and glossy. Fold in the drained pasta until everything is coated.
5. Line a 9 by 13 inch rimmed baking sheet with parchment or foil and grease it with butter or nonstick spray. Spread the warm mac and cheese out in an even layer. Grease a second piece of parchment and lay it greased-side down right on top. Refrigerate until cool and firm, about 1 hour. This is what lets you slice it.
6. Heat a large cast-iron or nonstick skillet over medium-low heat so the bread browns slowly while the inside heats through.
7. Stir together the 4 tablespoons softened butter and the garlic powder in a small bowl until smooth and spreadable.
8. Peel the top parchment off the chilled mac and cheese and cut it into 8 equal pieces. Each piece becomes one sandwich. (You can bag the pieces individually and refrigerate up to 3 days or freeze up to 1 month.)
9. Spread about 3/4 teaspoon garlic butter on one side of each bread slice. Lay 8 slices buttered-side down on a cutting board. Top each with a slice of cheddar, then a slab of mac and cheese (slide a spatula underneath and flip it on), then a slice of Colby Jack. Cap with the remaining bread, buttered-side up.
10. Melt a little of the grilling butter in the hot skillet and add as many sandwiches as fit without crowding. Cover and cook until the bottoms are 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 remaining sandwiches, adding butter as needed. Cut in half and serve hot.