01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking pan with cooking spray or butter.
02 - Combine cake mix, water, vegetable oil, and eggs in a large mixing bowl. Beat on medium speed for 2 minutes until completely smooth and well incorporated. Pour batter evenly into prepared pan.
03 - Bake at 350°F for 28-32 minutes until golden brown and a wooden toothpick inserted in center emerges clean. Remove from oven.
04 - Allow cake to cool in pan for 15 minutes. Using handle of wooden spoon, poke holes uniformly across surface at 1-inch intervals, pressing halfway through cake.
05 - Dissolve lime gelatin powder in 1 cup boiling water, stirring vigorously until completely dissolved. Stir in cold water and let mixture cool slightly.
06 - Slowly pour cooled gelatin mixture evenly over poked cake surface, ensuring liquid seeps into holes. Pour in circular motion for even distribution.
07 - Refrigerate cake uncovered for minimum 1 hour or until gelatin is completely set and cake is thoroughly chilled.
08 - In chilled metal bowl, beat heavy cream, powdered sugar, and vanilla extract with electric mixer on high speed until stiff glossy peaks form, approximately 3-4 minutes.
09 - Spread whipped cream evenly over chilled cake surface. Sprinkle with fresh lime zest and decorate with thin lime slices if desired.
10 - Cut into 12 equal rectangles. Serve immediately while cold or refrigerate until ready to serve.