Directions

1. Adjust oven rack to low position and heat oven to 400 degrees. Place potatoes on a baking sheet lined with parchment paper or foil. Bake until fork tender, about 45 to 60 minutes. Let cool slightly.
2. Handling the potatoes with a potholder, slice each in half lengthwise and scoop potato flesh into a blender or food processor - for an especially silky texture, use the blender - leaving a 14-inch border of flesh to support the potato skin. Puree scooped-out flesh, along with salt and pepper, until smooth. With machine motor running, gradually add both milks through feeder tube. Stop machine, add butter, then process until potatoes are silky smooth. (Puree and potato shells can be cooled, then refrigerated in an airtight container, up to 2 days.
3. Return to room temperature before proceeding.) Spoon puree back into each shell. Just before baking, press marshmallows into potatoes.
4. Bake at 400 degrees until potatoes are hot and marshmallows are golden brown, 10 to 12 minutes.