Combine the grapefruit juice, vodka and Simple Syrup in a bowl, then pour into a freezer-safe glass baking dish and pop in the freezer. Scrape with a fork every half hour until partially frozen and granular, about 3 hours. Serve in martini glasses and garnish with lemon zest. Eat with a spoon or a straw.
Combine the sugar and 1 cup water in a small saucepan and bring to a boil. Cook, stirring, until the sugar dissolves. Remove from the heat and let cool completely. Keep covered in the refrigerator up to 1 month.
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