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