Preheat oven to 425F. Scrub potatoes; pierce each with a fork. Place on oven rack; and bake until fork-tender- 50 to 60 minutes.
Remove potatoes from oven. Cool slightly; split in half lengthwise. Gently scoop out potato, keeping shells intact. Turn potato into a large bowl. Set 6 shells on a baking pan; discard remaining shells.
In saucepan, heat milk and butter slowly until butter melts-don't let mixture boil.
With portable electric mixer, beat potato until smooth. Add salt; gradually beat in warm milk mixture. Add egg yolk, beating well until mixture is light, fluffy and smooth. Spoon potato mixture into shells, making a swirl at the top. Return to oven. Bake 10 minutes, or until hot.
Meanwhile, make cheese sauce: In small saucepan, slowly heat butter; remove from heat; stir in flour, mustard, salt, pepper, cayenne and milk until smooth.
Before serving, spoon cheese sauce over top of each potato.
Makes 6 Servings
Recipe From: McCall's, April 1982 Joan Johnson