Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan or line it with parchment paper.
Prepare the Cake Batter: In a large bowl, combine the yellow cake mix, water, vegetable oil, eggs, lemon zest, and lemon juice. Using a hand mixer or stand mixer, beat on medium speed for about 2 minutes, until the batter is smooth.
Bake the Cake: Pour the batter into the prepared pan and bake for 28-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool completely.
Make the Lemon Pudding: In a separate bowl, whisk together the instant lemon pudding mix and 2 cups of cold milk. Whisk for 2 minutes until the pudding thickens.
Prepare the Whipped Cream: In a large bowl, beat the heavy whipping cream, powdered sugar, and vanilla extract until stiff peaks form.
Assemble the Cake: Once the cake has cooled, use a fork to poke holes in the top. Pour the lemon pudding evenly over the cake, allowing it to soak into the holes. Top with the whipped cream and spread it evenly over the cake.
Chill: Refrigerate the cake for at least 4 hours, or overnight, to allow the flavors to meld and the cake to set.
Serve: Slice into generous pieces and enjoy!