Start by preheating your oven to 350 degrees and prepare a 9×13 cake pan using a non-stick spray.
In a large bowl, pour in the dry cake mix, dry lemon pudding mix, lemon juice, eggs and melted butter. Then combine cake mix using a stand or hand electric mixer. Make sure the batter is completely combined by scraping the sides of the bowl with a spatula.
Pour batter into the prepared cake pan and smooth it out with a spatula. Batter will be thick and fluffy. Make sure the batter is even because it won’t level out on its own.
Bake for 20 minutes, then check doneness with a toothpick. If the toothpick doesn’t come out clean, bake for 3-5 minutes more, or until it’s set. Once fully baked, remove from the oven and set the pan aside to let it cool.
While the cake is cooling, begin making the glaze. In a large bowl pour in the 4 cups of powdered sugar. Next add the lemon zest and 3 tablespoons of lemon juice. Using a stiff whisk or a hand mixer at a low speed, stir these ingredients together. If needed, slowly add the last tablespoon of lemon juice. You’re looking for a consistency that is thick but will still pour nicely. If the icing becomes too thin and runs easily, add more powdered sugar until you get the consistency you want.
Once the cake has fully cooled, you can either pour the icing glaze over the cake while it’s still in the baking dish, or remove the cake from the pan, put it on a tray, cut it, then pour on and lightly spread the icing. If desired, top with lemon slices. Serve and enjoy!