Make Ahead: Prepare this recipe through Step 4, and freeze. Thaw potato mixture in the fridge at least 24 hours before you plan to serve, and let it stand 20 minutes before baking as directed.
Melt 2 Tbsp. butter in a large stockpot over medium-high heat. Add kale and green onions; stir to coat. Add broth; cover and cook, stirring often, 10 minutes or until tender. Add salt and pepper to taste. Transfer to a bowl; cover to keep warm.
Bring potatoes, 1 Tbsp. salt, and water to cover to a boil in stockpot over high heat. Reduce heat to medium, and simmer 20 minutes or just until potatoes are tender. Drain potatoes, and let stand 3 minutes or until dry. Return to stockpot. Mash with a potato masher until smooth; stir in cream cheese and 4 Tbsp. butter. Fold in kale mixture.
Microwave milk in a microwave-safe measuring cup at HIGH 1 to 2 minutes or until warm. Stir 1/2 cup warm milk into potato mixture. Add up to 1/2 cup more milk, 1 Tbsp. at a time, and stir until mixture thickens. (Mixture will firm up as it chills in Step ) Season with salt and pepper.
Transfer mixture to a greased (with butter) 2 1/2-qt. gratin dish or baking dish. Dot with remaining 2 Tbsp. butter. Cover dish tightly with plastic wrap, and then with aluminum foil. Chill 8 hours to 5 days, or freeze up to 2 weeks.
Preheat oven to 350°. Remove plastic wrap and foil, and bake 30 minutes or until thoroughly heated. Serve warm.
When oven space is at a premium, reheat these buttery mashed potatoes in the microwave. Make sure they're in a microwave-safe bowl, and microwave at HIGH in 1-minute increments, stirring each time, until hot. Then spoon them into a serving dish.
We tested with Swanson Natural Goodness 33% Less Sodium Chicken Broth.