In a medium saucepan combine sugar, cocoa powder, and gelatin. Gradually stir in evaporated skim milk. Cook and stir over medium heat until mixture comes to a boil and gelatin is dissolved. Cool slightly.
Stir yogurt and vanilla into chocolate mixture. Divide banana slices among 8 dessert dishes. Spoon the chocolate pudding over the banana slices. Cover and chill for 2 to 5 hours or until pudding is set. Garnish with mint, if desired. Makes 8 servings.