01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly.
02 - Prepare cake batter according to package directions using cake mix, water, oil, and eggs. Pour into prepared pan. Bake for 28-32 minutes until a toothpick inserted in the center comes out clean.
03 - Remove cake from oven and let cool for 10 minutes. Using the handle of a wooden spoon, poke holes all over the cake surface about 1 inch apart.
04 - Dissolve lime gelatin in boiling water, stirring until completely dissolved. Stir in cold water.
05 - Slowly pour the gelatin mixture evenly over the warm cake, allowing it to soak into the holes.
06 - Refrigerate cake for at least 1 hour until gelatin is completely set.
07 - Whip heavy cream with powdered sugar and vanilla extract until stiff peaks form. Alternatively, use prepared whipped topping.
08 - Spread whipped topping evenly over chilled cake. Garnish with lime zest and optional fresh lime slices. Keep refrigerated until serving.