1Heat oven to 350°F. Place paper baking cup in each of 12 regular-size muffin cups.
2In 10-inch skillet, melt 1 tablespoon butter over medium-high heat. Add rhubarb; cook 5 to 7 minutes or until softened. Add granulated sugar, the lime juice, ginger and cardamom. Remove from heat. Stir in raspberries, blackberries and the lime peel.
3In large bowl, stir together pound cake mix, rum, 6 tablespoons butter and the eggs. Fold in rhubarb-berry mixture. Spoon batter evenly into muffin cups.
4Bake 17 to 20 minutes or until cakes spring back when touched lightly in center. Remove cakes from pan. Cool completely, about 30 minutes. To serve, sprinkle cakes with powdered sugar.