1. Preheat oven to 180ºC degrees.
2. Cook macaroni in salted boiling water according to package directions. Drain and rinse well with cold water. Pour into a 3 litre casserole.
3. In a saucepan, melt butter and stir in flour. Gradually stir in milk and sour cream. Add salt and pepper.
4. Cook over low heat, stirring constantly, until sauce bubbles and thickens.
5. Reserve 1 cup grated cheese for the top of the casserole. Toss macaroni with remaining cheese.
6. Pour sauce over macaroni and mix thoroughly. Sprinkle with remaining cheese.
7. Bake for 1 hour, or until bubbly and browned.
8. Serve immediately.