Bring a large pot of salted water to a boil and cook the elbow pasta according to package instructions. Drain and wipe out the pot and return it to the stovetop.
Prepare the cheese:
Place the freshly grated cheddar cheese in a bowl with 1 tablespoon of cornstarch. Mix with your hands to coat the cheese. Set aside.
Prepare the mornay sauce:
Heat the butter in the pot over medium heat. Once melted and frothy, add the flour and stir to incorporate. Cook for 2 to 3 minutes until fragrant and golden brown.
Add a ladle of the milk and whisk to create a slurry. Continue adding the milk, one ladleful at a time, whisking after each addition until all the milk has been added, and the sauce is smooth and creamy.
Add the harissa paste:
Reduce the heat to medium-low. Stir in 2 tablespoons of harissa paste and whisk until smooth. Taste and add another tablespoon of harissa paste if you prefer a spicier sauce.
Melt the cheese:
Stir in the shredded cheddar cheese and whisk gently until the cheese is melted and the sauce is smooth. Add a splash of milk to thin the sauce out to your desired consistency and whisk until smooth.
Finish the sauce:
Taste the sauce and season with a pinch of salt, cayenne powder, and white pepper.
Finish the mac and cheese:
Add the cooked elbow pasta and stir to combine. Cook over medium-low for another minute or two to warm the pasta and coat it in sauce. Remove from heat.
To serve:
Divide mac and cheese between bowls. Serve with a pinch of cayenne powder or paprika on top if desired. Enjoy!