Pop your popcorn using your preferred method (air popper, stovetop, or microwave). Transfer to a large, heatproof bowl, removing any unpopped kernels.
Combine the Karo sypup and butter in a large microwave safe bow. Microwave until melted.
Add sugar and Jello powder. Put in the microwave and cook until it comes to a rolling boil (doubles in size).
Pour the hot candy syrup over the popcorn. Use a heatproof spatula or wooden spoon to gently toss and coat the popcorn evenly. Be careful—the syrup is very hot!
Spread the coated popcorn onto a parchment-lined baking sheet. Bake at 250° F for 30 minutes, stirring every 10 minutes.
Once cooled and hardened, break up any large clusters and serve in bowls or bag up for spooky party favors.