Saute leeks in tsp of butter until soft. Set aside.
Melt the remaining butter, add flour, and cook for 5 minutes over very low heat, stirring often. Do not let the flour brown. Remove from heat, and stir in the milk gradually, with a whisk or fork, to prevent lumps. Cook this for another five minutes. Add salt, pepper, cooked leeks, and cream. Melt three-quarters of the cheese into the sauce.
Mix sauce and cooked macaroni, and pour into a greased 9 x 9 in. pan. Top
with remaining cheese. Bake at 350-375 until the top and edges are slightly brown, 40 minutes or longer. Remove from oven, and let stand for 5 to 10 minutes before serving