Preheat oven to 450°F.

Prepare potatoes as package directs.

Remove stems from mushrooms; chop stems and set aside. Carefully scoop out center of each mushroom cap with a spoon, leaving 1/2-inch shell.

Melt butter in 10-inch skillet over medium heat. Add chopped mushroom stems and cook 3 minutes or until tender. Remove from heat and stir in mashed potatoes, 2 tablespoons chives and salt.

Fill each mushroom cap with potato mixture, mounding it slightly. Bake 10 minutes or until potatoes are lightly golden. Sprinkle mushrooms with remaining 1 tablespoon chives.