Bring a large pot of salted water to boil (I add about 1 Tablespoon of salt). Meanwhile, grate the cheese.
Add the macaroni to the water when it has come to a rolling boil. Cook for one minute less than the package instructions.
While the noodles are boiling, melt the butter in a large pot over medium heat. Add the flour and whisk until it is smooth. Cook the flour and butter for a minute (it will start to smell toasty).
Slowly whisk in the milk, salt and pepper and increase the heat to medium high. Continue whisking and cook until it thickens slightly (about 4 minutes). Once it has thickened slightly, turn the heat OFF.
Add the shredded cheese and whisk until smooth and fully melted. Stir in the harissa. Taste and add additional seasoning if needed.
Add the thoroughly drained macaroni to the cheese sauce and stir to combine. Sprinkle with chives and devour!
Macaroni has a bad reputation as leftovers. It tends to lose its creaminess overnight because the noodles absorb more of the moisture. A simple solution is to stir in a little bit of milk (2 Tablespoons or so) when you reheat it—that replenishes the moisture!