Preheat the oven to 375 degrees F. Spray an 8-inch square baking dish with nonstick cooking spray.
Put the mashed potatoes in a large mixing bowl. Stir in the vegetable stock, sour cream, onion dip powder and half of both the chives and bacon. Season generously with freshly cracked black pepper.
Pour the potato mixture into the prepared baking dish. Sprinkle the cheese evenly over the top. Bake until the cheese is melted and slightly golden and the casserole is bubbling around the edges, about 20 minutes. Sprinkle with the remaining chives and bacon. Serve warm with potato chips.