Pour 3 tablespoons cold water into a small bowl and sprinkle gelatin evenly over the surface. Let sit for 5 minutes until the gelatin softens and looks wrinkled - this is called blooming and ensures smooth results.

In a small saucepan, combine heavy cream, milk, sugar, and salt. Warm over medium heat, stirring occasionally, until steam rises and tiny bubbles form around the edges - about 5 minutes. Do not boil.

Remove from heat and whisk in the bloomed gelatin until completely dissolved, about 1 minute. Whisk in vanilla extract. Strain through a fine-mesh sieve into a large measuring cup with a spout.

Divide the warm cream mixture evenly among 6 ramekins or glasses, filling each about two-thirds full. Refrigerate uncovered until set, at least 4 hours or preferably overnight.

Combine diced mango, sugar, and lime juice in a blender. Puree until completely smooth, scraping down sides as needed, about 1 minute. Taste and add more sugar if your mangoes are tart.

Once panna cotta is fully set, spoon about 1/3 cup mango puree over each custard. The puree should sit on top without sinking. Return to refrigerator for 30 minutes to chill the topping.

Serve chilled in the ramekins, or to unmold: run a thin knife around the edge, dip the bottom in hot water for 10 seconds, then invert onto a plate. Garnish with mint if desired.