Step 1: Preheat the oven to 350 degrees F. Line a 13-by-9-inch baking pan with foil, leaving a 2-inch overhang on the long sides of the pan. Spray the foil with cooking spray.
Step 2: Mix the melted butter into the finely ground vanilla wafers and add 2 Tablespoons of water. Mix well and press the crumbs onto the bottom of the prepared pan, forming a crust. Bake the crust for 20 minutes and allow to cool.
Step 3: Set aside 2 cups of the watermelon juice in a small, microwave-safe bowl. In a large bowl, combine the remaining 3 cups of watermelon juice with the lemon zest, lemon juice, sugar and milk.
Step 4: Sprinkle the gelatin over the reserved 2 cups of watermelon juice. Allow to sit for 2 minutes so that gelatin can dissolve and begin to absorb liquid, or "bloom." After the gelatin blooms, heat it in the microwave for 2 minutes on high, or heat gently in a small pot over low heat on the stove top, until the gelatin has completely melted into the juice and the liquid is smooth.
Step 5: Add the watermelon juice with the melted gelatin to the larger bowl of juice from Step 3. Stir to combine. Gently pour the mixture over the prepared crust from Step 2 and place in the freezer for 2 hours or until the mixture is firm. (Alternately, place the pan in the refrigerator and allow to set for 3 to 4 hours.)
Step 6: Keep in the refrigerator until ready to serve. Before serving, cut the watermelon bars into squares. If desired, sprinkle each serving with confectioners' sugar or a dollop of whipped cream.
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