Preheat oven to 400°.
Melt butter in a large skillet and whisk in flour until smooth.
Cook, whisking constantly, 1 minute.
Gradually whisk in the warm milk, and cook, whisking constantly, 5 minutes tilll thickened.
Whisk in salt, black pepper, 1 cup of the shredded cheese.
Stir in pasta.
Spoon pasta mixture into a lightly greased 2-qt. baking dish
Top with the rest of the cheese and bake at 400° for 20 minutes.