In a small saucepan heat ricotta cheese until thinned, stirring con- stantly with wooden spoon; gradually add remaining cheeses, stirring constantly after each addition until cheeses are melted. Continuing to stir, add milk, 1 tablespoon at a time, stirring until thoroughly com- bined. Keep warm over lowest possible heat. In a mixing bowl combine hot pasta and butter, tossing until butter is completely melted. Transfer pasta to serving dish and top with cheese sauce. Toss to combine and serve immediately.
