Add up to 1/2 cup vanilla-flavored Greek Yogurt to the cooled mixture to make an opaque creamy gelatin.
Place 1 cup of juice in a small bowl and sprinkle with gelatin.
In a small saucepan heat the remaining juice until hot. Turn off the heat and stir in the gelatin mixture. Stir until well combined.
Pour mixture into 8x8-inch pan for thicker hearts or a 13x9x2-inch pan for thinner hearts. Refrigerate for 6 hours or overnight.
Coat heart-shaped cookie cutters with cooking spray. Cut gelatin into heart shapes. Slide an offset spatula under each heart and gently lift from the pan. Transfer to a plate and serve immediately or refrigerate, covered, for up to 2 days.