1. Remove husks from tomatillos. Bring large pot of water to a boil. When water is boiling, blanch tomatillos for 2 minutes to remove sticky residue from outside. Transfer tomatillos to bowl filled with ice water to stop cooking process. When cool, remove from water; set aside. 
2. Bring another large pot of water to a boil. Place potatoes in water. Boil until tender but not cooked through, about 10 minutes. Drain; remove from water. 
3. Heat grill to medium heat. Spray tomatillos with nonstick cooking spray. Grill tomatillos and potatoes on all sides until charred. Let cool. Place on cutting board. Slice tops off tomatillos. Chop into medium-sized pieces. 
4. Place tomatillos and potatoes on large sheet of foil. Top with butter and sour cream. Season with salt and pepper. Fold up sides to create a packet. Place on grill over medium heat. Cook for 5 minutes or until hot. Carefully open packet. Slide contents into bowl. Stir to combine before serving.

Calories: 282, Fat: 13 g (8 g Saturated Fat), Cholesterol: 28 mg, Sodium: 361 mg, Carbohydrates: 39 g, Fiber: 5 g, Protein: 6 g.