Line 2 12-cup muffin tins with paper liners. Prepare cupcakes using directions on cake mix box. Fill cups halfway. (Discard any leftover batter.) Bake until a toothpick inserted in center comes out clean, 15 to 18 minutes. Remove from pans and let cool on a wire rack.
Let ice cream soften slightly in a bowl. Fold in chocolate bars. Cover; freeze 1 hour.
Level cupcakes with a serrated knife. Put a scoop of ice cream on top of each cupcake; freeze until ice cream firms up but cake is still soft, about 30 minutes.
Whip cream and sugar until soft peaks form. Put a dollop on top of each cupcake. Drizzle with chocolate syrup; sprinkle with jimmies, if desired.