Preheat oven to 350 degrees and spray a 9×13 inch pan with non-stick spray.
Combine cake mix, oil, water, and eggs in a large bowl and whisk until well mixed. Continue stirring until there are no lumps.
Pour into a 9×13 pan and bake according to box directions, then allow the cake to cool to room temperature on a wire rack.
1 box lemon cake mix, 1 cup water, 1/2 cup vegetable oil, 3 eggs
Mix the lemon Jello mix and boiling water until the mix has dissolved.
1 box lemon jello mix, 1 cup boiling water
Using a fork, poke holes all over the top of the cake. Pour Jello mixture over top, then cover tightly with foil and move to the refrigerator to chill overnight.
Before serving, whisk together the powdered sugar, heavy cream, and lemon juice in a small bowl.
2 cups powdered sugar, 2 tbsp. heavy cream, 2 tbsp. lemon juice
Pour the mixture over the top of the lemon cake. If desired, sprinkle a bit of lemon zest on top.
Cut into squares and top with a lemon slice. Enjoy
lemon slices for garnish