How to Make It
In a large bowl, whisk together 2 1/4 cups prosecco and sugar. Sprinkle over gelatin and let sit for about 1 minute to dissolve. (This is important—if you stir the gelatin too soon, it won’t dissolve properly.)
Meanwhile, combine remaining prosecco and water (or vodka) in a small saucepan. Bring to a boil; remove from heat. Pour over gelatin mixture. Whisk mixture until gelatin is fully dissolved, about 3 minutes. Whisk in lemon juice and wheatgrass juice.
Pour mixture into a 13x9 baking dish and refrigerate at least 3 hours or until gelatin is set. When firm, cut gelatin with shamrock cookie cutters. Sprinkle over sanding sugar (if using) and serve immediately.
Find fresh wheatgrass juice at your local juice bar, or make it from scratch. To yield ½ cup juice for this recipe, simply combine 1 cup roughly-chopped wheatgrass (available at Whole Foods) and ¾ cup water in a high-powered blender. Blend until smooth; strain juice through a cheesecloth into a small bowl and use immediately.