Preheat your oven to 350°F and grease a 9x13-inch cake pan using a nonstick spray or you may line with parchment paper.
In a large bowl, combine cake mix, dry pudding mix, lemon juice, eggs, and melted butter using a stand or handheld mixer. Make sure to scrape down the sides of the bowl to fully incorporate the dry ingredients.
1 (15.25oz.) box yellow cake mix, ¼ cup instant lemon pudding, ¾ cup lemon juice, 4 large eggs, ⅓ cup salted butter
The batter will be fluffy and a bit thick. Pour the batter into your prepared cake pan and use a spatula to smooth and even out the batter. Make sure the batter is even throughout, because of its density, it won’t automatically level out.
Bake for 20 minutes and check for doneness using a toothpick. If the toothpick doesn’t come out clean, bake for 3 to 5 minutes more or until set. Remove from the oven and let the cake cool in the pan.
While the cake is cooling, make the icing glaze. In a large bowl add the powdered sugar, lemon zest, and 3 tablespoons of lemon juice. Use a stiff whisk or mixer to blend together and add the last tablespoon of lemon juice if needed. You want a spreadable icing that is thick but will still pour. If the icing becomes too thin and runs easily, add more powdered sugar until you achieve the right consistency.
4 cups powdered sugar, 4 tablespoons lemon juice, 2 tablespoons lemon zest
Once the cake has cooled, you can pour the icing glaze over it in the pan and cut it. Or you can remove the cake from the pan, place it on a serving tray, cut it, then pour it on and lightly spread the icing.
Top with lemon slices and serve.