Coat an outdoor grill or stove-top grill pan with cooking spray and preheat to medium-high.
In a small bowl, whisk together melted butter, brown sugar and cinnamon. Brush mixture all over flesh side of plums. Grill plums, flesh side down, 5 minutes, until soft. Serve plums with lemon sorbet. Garnish with candied peanuts and mint, if desired.