Slice the top third off each lemon, then cut a small section of rind off each root end (so lemons can stand upright). Hollow out the lemons, reserving juices for the granita. Freeze lemons 2 to 3 hours.
In a small saucepan over medium heat, combine sugar, juice, water, zest, and chopped lemon verbena leaves. Bring to a simmer, stirring occasionally, until the sugar dissolves. Remove from heat and cool to room temperature. Strain the mixture into a shallow baking dish and freeze 1 hour.
Remove baking dish from freezer, scrape the mixture with a fork, and return to freezer 15 minutes. Repeat cycle of scraping and freezing twice more, scraping one last time before filling frozen lemon cups. Garnish with whole lemon verbena leaves.