To roast garlic: Preheat oven to 400 degrees. Cut off the top ¼ of the head then place, cut side up, on a piece of foil. Drizzle olive oil over top then wrap up in foil. Roast in oven for about 40 minutes. When it is cool enough to handle, squeeze the garlic cloves out of their skins into a bowl. Mash with a fork to form a paste. Set aside.
Preheat oven to 375 degrees. In a large saucepan, melt three tablespoons of the butter over medium heat and stir regularly. The butter should bubble (but not pop or burn) and solids will form in the pan. Continue stirring until butter darkens and emits a nutty aroma. (total process is about 5 minutes) Add flour at this point to make the roux for your sauce.
Stir in the flour and cook the roux for about two minutes. Then whisk in milk slowly in order to prevent lumps. Bring mixture to a light boil then add salt, pepper, and roasted garlic. Stir until uniform.
Add cheese, about ½ cup at a time, mixing well between each addition. Stir until sauce is uniform in texture and color.
Add in cooked pasta and mix until it is evenly coated with sauce. Spread into a 13 by 9-inch baking dish.
In a small bowl, melt remaining butter (I just pop in the microwave), bread crumbs, and parsley to form a crumb mixture. Sprinkle over top of mac and cheese in baking dish.
Place dish in the oven and back for 20-30 minutes or until top is lightly browned and mac & cheese is bubbly.