Twice-Baked Potato

Two points opening here:
1) This is more a method than a recipe
2) Conceptually, this is actually really simple: It's a mashed potato that's been shoved back into its skin.

Ingredients:
Russet or other baking potatoes
Fat (preferably duck fat)
Butter
Egg
Salt
Pepper
Assorted other mix-ins and spices (e.g. bacon)

1) Preheat oven to bake at 400F
2) Rub the potatoes down with fat
3) Bake uncovered (I may revise this to wrapped in foil as I get more experience) until soft all the way through; for large potatoes (~3.5" diameter at widest point), that's somewhere between 1.5 and 2 hours
4) Once soft all the way through, cut the top ~1/3 off of each one and scoop the flesh into a bowl on the side
(4b - For smoother texture, pass through a potato ricer and, if desired, a drum sieve)
5) Add butter, seasonings, and mix-ins until satisfied with flavor. Usually this will mean a fairly large amount of butter and salt and pretty hefty dose of pepper.
6) Whip in egg (1 egg per 2-4 potatoes) and give a final taste test to adjust seasoning
7) Put the top of the potato skin back into the bottom of the potato, then spoon the mash back in, pressing down to get it into all of the nooks and crannies (consider putting a bit in before you put the top back on, to help maximize space efficiency). Texture the top and brush with melted butter if desired.
8) Bake for another 20-30 minutes.