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 completely smooth, approximately 2 minutes. Pour batter evenly into prepared pan.
03 - Bake for 28 to 32 minutes until a toothpick inserted into the center emerges clean. Remove from oven and allow to cool for 15 minutes.
04 - Using the handle of a wooden spoon, poke holes evenly across the entire surface of the cake, spacing approximately 1 inch apart.
05 - Dissolve lime gelatin completely in boiling water, stirring until no granules remain. Stir in cold water.
06 - Slowly pour the gelatin mixture over the warm cake, ensuring it fills the holes evenly.
07 - Refrigerate for at least 1 hour or until completely chilled and gelatin is fully set.
08 - In a chilled bowl, beat heavy cream, powdered sugar, and vanilla extract until stiff peaks form, approximately 3 to 5 minutes.
09 - Spread whipped cream evenly over the chilled cake. Sprinkle with fresh lime zest. Add optional thin lime slices for decoration.
10 - Slice into squares and serve cold. Store covered in refrigerator.