Start by setting out your jello shot cups on a tray so they're ready to fill. Having them arranged beforehand makes the pouring process much easier and less messy.
In a medium mixing bowl, combine the grape jello powder with ⅔ cup of boiling water. Whisk the mixture continuously until the jello granules completely dissolve and you have a smooth, uniform liquid with no visible powder remaining.
Add the cold water and grape vodka to the jello mixture. Give everything a quick stir to combine all the liquids evenly throughout the bowl.
Carefully fill each shot cup about ⅔ of the way full with the jello mixture. Leave some room at the top for your garnishes and to prevent spills when moving them to the refrigerator.
Place the filled cups in the fridge and let them chill for at least 2 hours until fully set. The jello should be firm to the touch and jiggly but not liquid. Once set, top each shot with a dollop of whipped cream and add a grape Jolly Rancher gummy on top if you're using them. Serve immediately and enjoy!
Notes
Don't skip the boiling water - Using water that's not hot enough won't dissolve the jello properly, leaving you with grainy, clumpy shots.
Measure your vodka carefully - Too much alcohol will prevent the jello from setting properly, leaving you with liquid shots instead of jiggly ones.
Use a frosting bag for neat presentation - If you want picture-perfect whipped cream dollops, pipe them on using a frosting bag with a star tip for a professional look.
Chill on a flat tray - Keep your cups level while they set by placing them on a baking sheet or flat tray before refrigerating to avoid lopsided shots.