Cook the macaroni according to the instructions.
Melt the butter in a large sauce pan. Add the flour. Gradually add the milk as the sauce thickens (reserve 1 cup). Add the cheese. Stir until the cheese is melted in. Scramble the eggs in a bowl and add the cup of milk (and the cornstarch) and mix into the cheese sauce. (The cornstarch is an old trick to keep the custard from separating.)
Mix in the cooked macaroni. Pour into a casserole dish. Sprinkle top with parmesan cheese.
Turn oven to 350 deg and bake for 1 hour.