Prepare cake batter and bake as directed on package for 2 (8 or 9-inch) round layers. Loosen cakes from sides of pans with knife; cool in pans 15 minutes.
Add boiling water to gelatin mix; stir 2 minutes until dissolved. Stir in cold water. Pierce cake layers with large fork at ½-inch intervals. Pour half the gelatin over each cake layer. Refrigerate 3 hours.
Mix sour cream and sugar in medium bowl; gently stir in Cool Whip. Dip bottom of 1 cake pan in warm water 10 seconds to loosen; invert cake onto plate. Carefully remove pan; spread cake with 1 cup Cool Whip mixture. Reserve a few cherries for garnish. Chop remaining cherries; sprinkle over cake on plate. Remove remaining cake layer from pan; place on first cake layer. Frost top and sides with remaining Cool Whip.
Melt chocolate as directed on package; cool slightly. Drizzle over cake. Garnish with reserved cherries. Store in refrigerator.