In a large bowl, combine 1 lb 4 oz (600 g) frozen mango, lemon juice, and 1/2 cup (120 ml) water. Set aside. Cube 2 oz (60 g) frozen mango into small cubes. Set aside. In a small bowl, mix gelatin and 2 Tbsp water, and let sit for 5 minutes.
Heat milk and sugar in a small pot at medium heat until hot, but not boiling. Add gelatin mixture and stir until completely dissolved. Remove from heat. Cool to room temperature.
Process mango mixture in a food processor to make mango puree. Pour the mango puree in a pot, heat, and let boil for a few seconds. Sieve through a fine mesh strainer (discard the remaining fiber). Cool to room temperature.
Take 14 oz (400 g) of cooled mango puree, slowly pour in the milk mixture gently whisking, and combine together. Mix in heavy cream.
Divide the mixture into 4 glass cups. Refrigerate for 2 hours.
Mix mango cubes and the rest of the mango puree to make a sauce.
Remove chilled Mango Purin from the refrigerator. Pour the sauce over chilled Mango Purin and garnish with mint leaves.