Place potatoes in large saucepan; pour in enough cold water to cover and season generously with salt. Bring to boil; cook for about 15 minutes or until tender. Drain well and let cool until easy to handle.
Cut each potato in half; scoop out pulp, leaving small border all around to create hollow cup. Mash together potato pulp, milk, butter, salt and pepper until smooth. Fold in Monterey Jack, pepper Jack, jalapeños and green onions.
Preheat oven to 375°F. Spoon potato mixture into potato cups. Arrange on foil-lined baking sheet; sprinkle with Cheddar cheese.
Bake for 8 to 10 minutes or until heated through and cheese is melted. Dollop with sour cream and sprinkle with chives.
Tip: Use a melon baller or demitasse spoon to scoop out pulp from cooked potatoes.