Cook the pasta: Bring a large pot of salted water to a boil. Cook the pasta about 1–2 minutes less than the package directions suggest so it remains slightly firm. Drain well and do not rinse.
Cook the bacon: Add chopped bacon to a cold skillet and cook over medium heat until browned and crisp. Transfer it to a paper towel-lined plate, leaving about 1 tablespoon of bacon fat in the skillet.
Brown the mushrooms: Add the sliced mushrooms to the skillet and cook until their moisture evaporates and the edges become nicely browned. Add the cooked chicken and warm for a minute or two, then set aside.
Make the roux: Melt the butter in a saucepan over medium-low heat. Whisk in the flour and cook for about 1 minute until smooth and lightly foamy.
Build the sauce: Gradually whisk in the milk. Continue cooking and stirring until the mixture thickens enough to lightly coat a spoon.
Season the sauce: Add black pepper, garlic powder, optional cayenne, and a small amount of salt. Remember that bacon, cheddar, and Parmesan already contain salt.
Melt the cheese: Remove the saucepan from high heat. Add the cheddar, Monterey Jack, and Parmesan gradually, stirring until each addition melts into a smooth sauce.
Combine the pasta: Add the drained pasta to the cheese sauce and gently fold until completely coated.
Add the fillings: Fold in the browned mushrooms, cooked chicken, and most of the crispy bacon. Reserve a little bacon for finishing.
Prepare the topping: Mix the panko breadcrumbs with melted butter, cheddar, and Parmesan until the crumbs are lightly coated.
Assemble: Transfer the macaroni and cheese to a lightly greased baking dish and spread it evenly. Sprinkle the panko mixture over the top without pressing it down.
Bake: Bake at 375°F for about 15–20 minutes, or until the edges are bubbling and the panko topping is golden brown.
Rest and finish: Let the mac and cheese rest for about 5 minutes. Sprinkle with the reserved bacon and finish with a small drizzle of truffle oil.
