Preheat your oven to 400°F (200°C). Line a baking tray with aluminum foil, then place the bacon side-by-side on the tray.
Cook the bacon for 10-15 minutes or until it's reached your desired level of crispiness. Make sure to check on it after about 10 minutes, as some ovens cook faster. Remove the tray from the oven and transfer the bacon with tongs to a paper towel-lined plate.
Chop up the bacon and then set aside 2 tablespoons (30g) of the bacon grease, if using.
Bring a large pot of lightly salted water to a boil. Add the macaroni and cook for 7 minutes or per package directions. Drain the noodles, rinse them with cold water, and set aside.
Add the bacon grease (or butter) to a large saucepan over medium heat. Whisk in the flour to create a roux, cooking until bubbly. Add the milk in a thin stream while whisking. Season with salt, pepper, garlic powder, cayenne pepper, and nutmeg, then bring to a simmer and cook for 2 minutes.
Remove from the heat and mix in the cheese. Once the cheese is melted, fold in the macaroni and chopped bacon until evenly combined. It should look creamy and thick; if it looks a little soupy, continue cooking over medium heat until it thickens up more. (If your noodles are stuck together, just give them another quick rinse under cold water to loosen them up).
Spread the bacon mac and cheese evenly into a 9x9-inch baking pan lined with plastic wrap or parchment paper, cover with plastic wrap, and refrigerate for at least 3 hours, or overnight.