Mise en place (always!). Preheat oven to 350°. Grease a medium glass baking dish.
Peel and cube potatoes. Place potatoes in a large pot, add water until potatoes are covered. Bring to a boil. Lower heat slightly and continue simmering for 18-25 minutes, until potatoes are fork-tender.
While potatoes are cooking, cook bacon. Cool, press grease out, and crumble. Set aside.
Drain potatoes. Using a mixer, whip until smooth.
Cut cream cheese into smaller cubes. Add to potatoes. Add sour cream, butter, milk, salt, pepper, and chives. Mix well.
Fold in 1/2 of the bacon, 1/2 of the sharp cheddar, and 1/2 of the mild cheddar until well-mixed.
Spoon potato mixture into greased baking dish. Top with remaining half of cheeses and remaining half of bacon.
Bake at 350° for 20-30 minutes until heated through and top melted and slightly golden brown.
Garnish with chives and/or green onions, as desired.