Preheat oven to 350°F and spray a 9x13 inch pan with non-stick spray.
Then, in a large mixing bowl, whisk together cake mix, water, oil, and eggs until there are no lumps.
Pour into the prepared baking dish and bake according to box directions.
When done, allow the cake to cool in the pan.
Combine lemon jello mix and boiling water. Set aside.
Poke holes on top the cake using a fork. Then, slowly pour jello mixture over the top of the lemon cake.
Cover tightly with foil, and move to the refrigerator to chill overnight.
Before serving, whisk together powdered sugar, lemon juice, and heavy cream for the icing. Pour on top of the lemon cake and garnish with fresh lemon slices.
Notes
Refrigerate leftover cake in an airtight container for up to 4 days.