Using a medium pot, bring the water to a rolling simmer
Using a medium bowl, whisk together the boiling water the jell-o, and Rum
Fill all the shot cups ¾ way full and place them onto a cookie sheet into the fridge for a few hours to firm up
Garnish with Lime wedge, Mint, and fresh whipped cream
Notes
Make sure the water is at a full rolling boil. Hot water won't cut it here — you need a proper boil to fully dissolve the gelatin. Undissolved granules will leave your shots with a grainy texture instead of that smooth, clean set you're after.
Let the mixture cool slightly before adding the rum. Give the boiled jello mixture about 5 minutes to cool down before stirring in the Bacardi. Adding alcohol to very hot liquid causes some of it to evaporate, so a short rest keeps more of that rum flavor in the final shot.
Add the Sprite last and stir gently. Sprite is a carbonated mixer, so stirring too aggressively will knock out all the bubbles. A gentle stir after adding it keeps a little of that fizzy citrus lift in each shot.
Fill the cups only ¾ of the way full. This leaves room for the whipped cream garnish without making a mess, and it also means the shots are the right size — easy to pop in one go.
Use a cookie sheet or baking tray to move the cups to the fridge. Trying to carry individual shot cups is a disaster waiting to happen. Line them all up on a sheet pan before filling so you can slide the whole batch into the fridge in one easy move.
Wait until just before serving to add the garnishes. Whipped cream will deflate and go watery if it sits on the shots for hours. Add the lime wedge, mint leaf, and whipped cream right before you put them out — they'll look much better and stay fresher.
Make them the night before for the best set. While 3 hours in the fridge is the minimum, overnight is even better. The shots will be firmer, easier to garnish, and ready to go when your guests arrive.