Preheat oven to 425 degrees F or 350 degrees F. Scrub potatoes with a brush; pat dry. Prick potatoes in 2 or 3 places with a fork. (If desired, for softer skins, rub potatoes with shortening or wrap each in foil.)

Place potatoes in a shallow baking pan. Bake potatoes, uncovered, in a 425 degree F oven for 40 to 50 minutes for small potatoes or 60 to 70 minutes for large potatoes (or in a 350 degree F oven for 60 to 70 minutes for small potatoes or 80 to 90 minutes for large potatoes) or until tender. Remove from oven; cool slightly for easier handling.

For small potatoes, cut a thin crosswise slice off both ends of each baked potato. (For large potatoes, cut potatoes in half crosswise. Cut a thin crosswise slice from the round ends of the potatoes.) Scoop pulp from slices; discard skin. Place pulp in a bowl. Carefully scoop pulp from each potato, leaving a 1/4-inch shell. Add pulp to bowl; set shells aside.

Mash the potato pulp. Stir in sour cream or yogurt, cheese, chives, mustard, salt, and pepper. (If necessary, add 1 to 2 tablespoons milk to make of desired consistency.) Stir in hard-cooked eggs. Carefully spoon potato mixture into shells. Stand the potato "eggs" upright in a 2-quart rectangular or square baking dish.

Bake large or small potatoes, uncovered, in a 425 degree F oven about 20 minutes (or in a 325 degree F oven for 35 to 40 minutes) or until heated through and tops are lightly browned. Top with additional whole fresh chives, if desired. Makes 8 servings.

Prepare potatoes through step 4; chill. Reheat with ham at 325 degrees F for 45 to 50 minutes.