Place silicone heart mold (or molds) on a quarter sheet baking tray. Spritz with a thin layer of nonstick cooking spray. Use a paper towel or soft cloth to wipe away any excess.

In a small microwave-safe bowl, whisk together Jello powder, unflavored gelatin, and cold water. Continue whisking until powder dissolves.

Allow mixture to sit at room temperature for 10 minutes, then microwave on medium-high power for 60 seconds. Whisk with a small silicone whisk, then microwave 30 seconds more.

With an eye dropper, drop Jello mixture into prepared silicone heart mold.

Refrigerate 30 minutes, or until firm. (If Jello mixture begins to thicken in the bowl, reheat in microwave in 20-second increments as needed.)

Pop hearts from silicone mold onto a sheet of wax paper or parchment, or a silicone baking mat.

Store gummy hearts in a large plastic food storage bag or airtight container in the refrigerator for up to 2 weeks.