In a small saucepan over medium heat, melt butter, cream cheese and milk together. Blend until smooth and creamy. Keep warm.
In a separate 1 quart saucepan, bring two cups water to a boil. Add Idahoan Buttery Style Mashed Potatoes and whisk until well incorporated. Turn heat to low.
Pour in the cream cheese sauce and whip with a whisk until fluffy.
When potatoes are smooth and creamy, add mozzarella and cheddar cheeses and chives. Stir again. Let cheese set for 2 minutes.
Stir again, adjust salt if needed, and serve hot.
