1. Light a grill. In a medium bowl, combine the strawberries and blueberries with the lemon zest, lemon juice, sugar and cornstarch. Spoon 1/4 tablespoon of the butter in the center of each sheet of foil and top with the fruit. Bring 2 sides of the foil up over the fruit and fold to form a seam across the top. Fold the remaining 2 sides to seal the hobo packs completely.
2. Grill the hobo packs over moderate heat until the fruit is sizzling, about 10 minutes.
3. Spread the remaining 1 tablespoon of butter on the pound cake and grill on both sides just until toasted, about 1 minute. Transfer to plates. Open the hobo packs and pour the berries and juices over the pound cake. Serve with a dollop of crème fraîche or ice cream.