For the baked potato croquettes: Preheat the oven to 350 degrees F. Wash and bake the potato on a rimmed baking sheet or large pot, until fork tender, about 45 minutes. Cool and put through a ricer in a large bowl. Add the butter, bacon, sour cream, milk, green onion, salt, pepper. Mix together and form 4 balls. Place 1 piece of cheese in the center of each ball.
For the egg wash: Whisk the egg and milk together in a shallow bowl. Place the flour in a second shallow bowl. Roll the balls in the flour and then the egg wash, then again in the flour.
For frying: Heat the canola oil to 350 degrees F in a deep fryer or large pot. Deep fry the croquettes in batches in the oil until golden brown and hot in the middle, 3 to 4 minutes.