1. Remove rind from lemon using a vegetable peeler, avoiding white pith. Squeeze 3 tablespoons juice from lemon. Combine rind, 1/2 cup milk, half-and-half, sugar, and salt in a small saucepan; bring to a simmer over medium heat (do not boil). Remove pan from heat; cover and let stand 20 minutes. Discard rind. Sprinkle gelatin over remaining 3 tablespoons milk in a small bowl, and let stand at least 10 minutes. Return half-and-half mixture to medium heat; cook for 1 minute or until very hot. Add the gelatin mixture, stirring with a whisk until dissolved (about 1 minute). Stir in buttermilk and 3 tablespoons juice. Divide mixture among 4 (6-ounce) ramekins or custard cups coated with cooking spray. Cover and refrigerate for 4 hours.
2. Run a knife around outside edges of panna cotta. Place a plate upside down on top of each cup; invert onto plate. Garnish with mint and rind, if desired.