Place potatoes, cut side down on a foil or parchment-lined baking sheet. Bake until fork tender, about 30 minutes. Let cool slightly. Holding the potato in a pot holder-protected hand, scoop potato flesh into a blender canister or food processor bowl, leaving a 1/4-inch border of flesh to support skin. With machine motor running, gradually add both milks through feeder tube. Stop machine, add butter, then process until potatoes are silky smooth. Spoon a portion of puree back into each potato shell. Sprinkle a portion of marshmallows over each potato.
Bake until potatoes are hot and marshmallows are golden brown, 10 to 12 minutes.
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