Lightly coat a square or cube-shaped mold with nonstick cooking spray.

Pour the cold water into a small bowl and sprinkle the gelatin over the top. Let it bloom for 5 minutes.

In a saucepan over medium heat, combine the lemonade or white grape juice and sugar. Stir until the sugar dissolves, then remove from the heat.

Whisk the bloomed gelatin into the warm juice mixture until completely dissolved.

Stir in the vanilla extract, citric acid or lemon juice, and enough green food coloring to give the mixture a pale gelatinous-green color.

If using, stir in the edible glitter for a shimmering effect.

Pour the mixture into the prepared mold and tap gently to release any bubbles.

Refrigerate for at least 4 hours, or until fully set and firm enough to unmold.

Carefully unmold the gelatin cube onto a serving plate. Slice into cube-like portions if desired, or serve whole for a dramatic effect.