Dinner

Bring a medium saucepan of generously salted water to a boil and cook the macaroni until al dente, 8 to 10 minutes
Whisk together the plain flour, mustard powder, garlic powder, kosher salt, black pepper, and cayenne in a small bowl
Drain the pasta, return the empty pan to low heat, add the butter and when melted whisk in the flour mixture and cook until it begins to brown
Slowly whisk the whole milk and heavy cream into the flour mixture, cook until slightly thickened, remove from heat, gradually add the Monterey Jack cheese until melted, then stir in the drained cooked pasta
Line a rimmed baking sheet with parchment or foil, slick it with butter, pour the warm mac and cheese onto the sheet, cover with oiled parchment, and refrigerate until firm
Heat a large cast-iron or nonstick skillet over medium-low heat
Stir together 4 tablespoons butter and the garlic powder in a small bowl to make garlic butter
Remove the chilled mac and cheese from the refrigerator, peel off the top parchment, and cut into 8 equal pieces
Spread garlic butter on one side of each bread slice, place half the slices buttered-side down, top each with one slice of Cheddar, one piece of mac and cheese, and one slice of Colby Jack, then finish with the remaining bread slices buttered-side up
Place as many sandwiches as fit in the pan, cover and cook until the bottoms are browned, then turn and cook until the second sides are browned and the cheese and mac are heated through
Repeat with remaining sandwiches, cut in half if desired, and serve