How to Make It
In a small saucepan, cook the water with the sugar over moderate heat until the water is simmering and the sugar dissolves, about 4 minutes. Sprinkle the gelatin over the sugar syrup and whisk constantly over low heat until the gelatin has dissolved, about 1 minute.
Put the lime juice and mint leaves in a medium heatproof bowl and pour in the sugar syrup. Let steep for 15 minutes. Stir in the rum. Strain the mojito mixture into a large measuring cup, pressing on the mint leaves with the back of a spoon to extract as much liquid as possible.
Arrange 30 small paper cups on a rimmed baking sheet. Carefully pour the mojito Jell-O mixture into the paper cups and refrigerate the shots until they are chilled, about 3 hours. Serve cold.