1 stick butter
2/3 c. sour cream or light cream
3/4 c. grated Parmesan cheese
1 egg yolk, slightly beaten
2 tbsp. chopped parsley (optional)

Cook pasta. Meanwhile over low heat, melt butter. Add sour cream, then egg yolk. Add cheese. Heat until hot but not boiling. Toss with pasta. Garnish with parsley.