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While Peach or Watermelon flavors are the ones favored for a more realistic look, we chose Tropical Explosion. Jello has a "flavor finder" on it's website to help find other suitable flavors by color. We greased our mold with coconut oil instead of spray-on cooking oil, and used blue food coloring to lightly paint the veins in the mold before adding the Jello mixture.
While the recipe calls for leaving the jello-filled mold in the refrigerator overnight, we found that our heart was ready in a few hours. We also found that the recipe is almost enough for two hearts. Fortunately I own two of these gelatin molds.
(the heart pictured below is the second one made with the left-over mixture.)
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We give this creation two thumbs-up for being tasty and having just the right amount of gross-out factor!