Preheat the oven to 350 degrees. Prepare a 9X13 inch cake pan using nonstick spray. You can also use parchment paper to line the cake pan as well.
In a bowl, mix the dry pudding mix, lemon juice, cake mix, eggs, and melted butter using an electric mixer. Use a spatula and scrape down the sides of the bowl as you mix.
Pour the batter in the cake pan (or cake pans if you're making double) and smooth it out. It needs to be level before adding it into the oven.
Bake the cakes for 20 minutes and then check that the cake is done. If it's not done, bake for another 3-5 minutes. Once done, remove the cake and let it cool in the pan.
As the cake is cooling down, it's time to make the glaze. Mix the lemon zest, powdered sugar, and 3 T of the lemon juice in a bowl. Use a whisk or mixer to mix together and then add in 1 T of lemon juice if you need to think it out some. You want it to be easy to spread in a thin layer.
Once the cake has cooled down, pour the icing over the top and then cut the cake. You can also take the cake out of the pan completely and then cut and pour on the glaze. It's up to you.
Add some lemon slices on top and enjoy!