01 - Line an 8x8-inch baking pan with parchment paper, allowing overhang on all sides for easy removal of the frozen bars.
02 - Combine watermelon cubes, lime juice, lime zest, and sugar in a blender. Blend until completely smooth and no fruit chunks remain.
03 - Taste the mixture and adjust sweetness or lime intensity according to personal preference.
04 - Pour the smooth puree into the prepared pan. Tap the pan gently on the counter to release air bubbles and level the surface.
05 - Place the pan in the freezer for 4 hours or until completely firm and set throughout.
06 - Lift the frozen block from the pan using the parchment overhang. Use a sharp knife to cut into 12 even bars, wiping the knife clean between cuts if needed.
07 - Top bars with fresh chopped mint and lime wedges immediately before serving, if desired. Serve frozen.