01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly with cooking spray or butter.
02 - Combine cake mix, water, vegetable oil, and eggs in a large bowl. Beat according to package directions until smooth, approximately 2 minutes. Pour batter into the prepared pan.
03 - Bake for 28-32 minutes until a toothpick inserted in the center comes out clean. Remove from oven and let cool in the pan for 10 minutes.
04 - Using the handle of a wooden spoon, poke holes evenly across the entire surface of the warm cake, spacing holes about 1 inch apart.
05 - Dissolve lime gelatin completely in 1 cup boiling water. Stir in 1/2 cup cold water and mix well.
06 - Pour the gelatin mixture slowly and evenly over the warm cake, ensuring it fills the holes. Allow to cool slightly, then refrigerate for at least 1 hour until gelatin is completely set.
07 - In a chilled bowl, whip heavy cream with powdered sugar and vanilla extract using an electric mixer until stiff peaks form, approximately 3-5 minutes.
08 - Spread whipped cream evenly over the chilled gelatin-soaked cake. Sprinkle lime zest across the top and arrange fresh lime slices decoratively if desired.
09 - Refrigerate the completed cake for at least 30 minutes to allow flavors to meld. Serve cold and store leftovers refrigerated.