The combination of buttery Yukon gold and fluffy baking potatoes offers richness and a nice contrast both in flavor and texture. If you don't have a food mill, cream the potatoes and cooking liquid with a potato masher.
1 1/2 pounds Yukon gold potatoes, peeled and cut into 1/2-inch-thick slices
1 1/2 pounds baking potatoes, peeled and cut into 1/2-inch-thick slices
Place potatoes, garlic, and 1/2 teaspoon salt in a large saucepan, and cover with water. Bring to a boil. Reduce heat; simmer 15 minutes or until tender. Drain in a colander over a bowl, reserving 1/2 cup cooking liquid.
Press potato mixture in batches through a ricer into a large bowl. Stir in reserved 1/2 cup cooking liquid, remaining 3/4 teaspoon salt, and cream cheese.
Spoon potato mixture into a broiler-safe 11 x 7-inch glass or ceramic baking dish coated with cooking spray. Bake at 350° for 20 minutes or until thoroughly heated.
Combine Parmigiano-Reggiano and panko; sprinkle evenly over top of potatoes. Broil 4 minutes or until golden brown. Sprinkle with chives.
Make-ahead tip: Assemble this dish a day ahead, and bake shortly before serving.