Bake in the preheated oven until a paring knife easily inserts into the center of a potato, about 1 hour. Set aside to cool until easy to handle, about 10 minutes.

Combine potato flesh with butter and green onion; stir until butter is melted evenly into potato. Add salt, black pepper, cayenne pepper, and cheese; stir until cheese is melted. Pour cream and egg yolk into potato mixture; mix to combine. Season with salt to taste.

Bake stuffed potatoes until cheese is molten, 20 to 30 minutes.