Preheat oven to 350°F and line a 9x13-inch pan with parchment paper or spray with non-stick cooking spray.

In the bowl of a stand mixer, fitted with the paddle attachment, or with a hand mixer in a large bowl, beat together the cake mix, pudding mix, sour cream, oil, eggs, vanilla, and water on medium speed until well combined, about 2 minutes. Be sure to scrape the bottom of your bowl as needed.

Spoon batter into the prepared pan and use an offset spatula or spoon to spread the batter into an even layer.

Bake in preheated oven for 35-40 minutes, or until golden brown and the top of the cake springs back when lightly touched.

While the cake is baking, make the filling: In a large microwave-safe bowl, whisk together the sugar and eggs until smooth and light in color. Whisk in the lemon zest, lemon juice, and melted butter until well combined.

Microwave on high in 1-minute increments for 4-5 minutes total, whisking after each minute, until the curd has thickened and coats the back of a spoon. Towards the end, you may want to microwave in 30-second intervals, depending on your microwave.

Use a spatula to push the curd through a fine mesh strainer into a bowl or measuring cup. Set aside.

When the cake is finished baking, allow to cool for 10 minutes. Using a smoothie straw or the handle of a wooden spoon, poke holes across the cake, spaced every inch or so, halfway into the cake.

Pour the lemon curd evenly over the cake, smoothing it with a spatula and making sure it gets into the holes. Allow cake to cool completely.

Evenly spread the whipped topping onto the cake. Cover and chill for at least 4 hours. If desired, top with lemon zest or lemon slices before serving.