Place raw potato cubes in a large steamer basket over boiling water. Cover and steam until very tender, 25 to 30 minutes.
When potatoes are nearly tender, preheat oven to 350˚F. (see recipe note #1)
Transfer tender potatoes to a large bowl. Add butter, salt, and pepper; mash until nearly smooth with a handheld masher. Add milk, half of the cheese, bacon, sour cream, and chives; mash until smooth.
Scoop potato mixture into prepared baking dish; sprinkle with remaining cheese and bake, uncovered, until filling is heated through and cheese is melted, 35-40 minutes. Serve hot.
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Notes
You can make these ahead through spreading in the prepared baking dish. Let cool then cover and refrigerate. When ready to bake, top with remaining cheese and bake at 350˚ for 40-45 minutes.