How to Make It
In a small saucepan, sprinkle the gelatin over ½ cup of the buttermilk; let stand until dissolves, about 5 minutes. Cook over medium low heat, whisking constantly, until hot and the gelatin is dissolved, about 3 minutes. Transfer to a medium bowl and whisk in the remaining 1 ½ cups buttermilk, the 1/3 cup agave and salt. Pour the mixture into four 6-ounce bowls or glasses and refrigerate until well chilled and set, about 2 hours.
Hold the mango upright on a work surface. Using a knife, cut on either side of the flat pit to remove the mango cheeks, then trim around the pit to remove any remaining flesh; discard the pit. With one hand, hold a thick pint glass firming in place on a work surface. Using your other hand, place the mango on the rim of the glass where the fruit meets the skin and, with gentle pressure, push the mango cheek down the side so the flesh falls into the glass and the skin remains in your hand; discard the skin. Cut the peeled mando into 1/4 inch dice.
Top the panna cottas with the diced mango, small mint leaves and a drizzle of agave nectar. Serve chilled.