The Ultimate David Sandwich: Mac & Cheese Grilled Cheese

Directions

Melt the butter in a medium-size saucepan. Add the flour and cook, while stirring, for 30 seconds.
Add the milk and bring to a simmer, while stirring constantly. When the sauce begins to simmer, remove the pan from the heat and add the shredded cheese, mustard, salt, and pepper. Stir until all of the cheese has melted. Add the bacon and the cooked elbow macaroni to the cheese sauce, and stir to fully coat the macaroni. Set aside.
To prepare the sandwiches, preheat the oven to 350°F. Place 8 slices of bread on a work surface and spread the mayonnaise onto one side of each slice. Flip over 4 slices of the bread and place 1 slice of cheddar on each.
Divide the macaroni over the cheese-covered bread slices and spread evenly. Top the macaroni and cheese with the remaining slices of cheddar, and cover with the remaining bread slices, mayonnaise side facing out.
Preheat a square griddle pan to medium heat. Place the sandwiches on the hot griddle and toast until golden brown on one side, about 5–8 minutes. Flip the sandwiches, then place the griddle, with the sandwiches, in the oven and bake for about 5–8 minutes, or until golden brown and the cheese slices have melted.