Stuffed Baked Potatoes With Cheese Sauce

• 6 shells on a baking (pan)
• 4 potatoes
• 1 egg yolk
• 350 ml milk
• 110 gr sharp cheddar cheese (grated)
• 55 gr butter
• 16 gr flour
• 2 ml dry mustard
• 1 gr salt
• 1 gr salt