Adjust oven to low position and heat oven to 400 degrees. Place potatoes, cut side down on a silpat-lined baking sheet. Bake until fork tender, 30 to 40 minutes. Let cool until warm enough to handle. (Potatoes can be left, cut side down on baking tray in a cool spot over night.) Scoop flesh into a microwave-safe bowl, leaving a 1/ 4-inch border of flesh to support skin, and microwave potato flesh back to a hot state. Turn potatoes into a blender canister or food processor bowl; add yogurt and/or milks and process, stopping and stirring as necessary, until pureed. Add butter, then process until potatoes are silky smooth; season to taste with salt and pepper. Spoon a portion of puree back into each potato shell. (Can sit at room temperature for a couple of hours.) When ready to bake, sprinkle a portion of pumpkin seeds over each potato. Bake until heated through, 10 to 12 minutes.