Preheat your oven to 180 C (350 F). Get out a 9x9 baking dish.
Open and drain the cherries into a measuring cup. Set the cherries aside and keep that liquid handy for the next step.
In a sauce pan, add the sugar and cornstarch, whisk to mix together. Whisk in the liquid from the cherries. Place on medium heat and whisk continuously until mixture comes to a boil and thickens.
Remove from heat and stir in butter, lemon juice, and cherries. Set this aside.
In a mixing bowl, add the Jiffy yellow cake mix, egg, and cold water, stir to mix together, then beat for 3-4 minutes with an electric mixer on medium or 5-6 minutes by hand.
Pour the cherry mixture into the baking dish and spread out evenly.
Spoon the cake mix over the cherry mixture.
Bake for 35-40 minutes or until a toothpick inserted in the cake comes out clean. Remove from the oven and place on a rack to cool.
Cut into squares and serve with whipped topping if desired, enjoy.