1. Prepare the Chocolate Cake: Preheat your oven according to the cake mix box instructions. Prepare the cake batter using the eggs, oil, and water as directed. Bake the cake until a toothpick inserted into the center comes out clean. Let it cool completely, then crumble it into small pieces.
2. Prepare the Cherry Topping: In a saucepan, combine the drained sour cherries and granulated sugar. Cook over medium heat until the sugar dissolves and the cherries begin to soften. In a small bowl, whisk the cornstarch with 2 tablespoons of water to form a slurry. Add the slurry to the cherries and stir continuously until the mixture thickens. Remove from heat and let it cool completely.
3. Whip the Cream: In a large, chilled bowl, whip the cold heavy whipping cream with an electric mixer until soft peaks form. Gradually add the powdered sugar and vanilla extract, and continue whipping until stiff peaks form. Be careful not to over-whip.
4. Assemble the Dessert Cups: In individual serving glasses or jars, create layers. Start with a layer of crumbled chocolate cake, followed by a layer of whipped cream, and then a spoonful of the cooled cherry topping. Repeat the layers until the glasses are filled, ending with a layer of whipped cream.
5. Garnish and Chill: Garnish the tops with chocolate shavings or a dusting of cocoa powder, if desired. Cover the dessert cups and refrigerate for at least 1 hour to allow the flavors to meld and the dessert to chill thoroughly before serving.