Choose watermelon that is naturally sweet and deep red in color. Seedless is preferable, but, if necessary, strain the puree to eliminate any bits.
Mix the watermelon with a squeeze of lime juice and a spoonful of sugar (if necessary). Strain the mixture to take out pulp for a clean, frosty ending.
Freeze in a shallow pan and scrape with a fork every 30 minutes for 2 hours. This provides you with a soft, spoonable sorbet without an ice cream machine.
Store the chocolate bar in the refrigerator before shaving or grating it. Cold chocolate curls better and provides a light snap to the creamy sorbet.
To add some playful presentation, fill a small watermelon shell with scoops of the sorbet. Top with shavings of chocolate just before serving for a chilled, party-perfect appearance.