01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly.
02 - In a large bowl, combine cake mix, water, vegetable oil, and eggs. Beat according to package directions until smooth. Pour batter into prepared pan.
03 - Bake for 28-32 minutes, or until a toothpick inserted into the center comes out 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 cake surface, spacing approximately 1 inch apart.
05 - Dissolve lime gelatin in boiling water, stirring until completely dissolved. Add cold water and mix well.
06 - Slowly pour the gelatin mixture over the warm cake, allowing it to seep into the poked holes.
07 - Refrigerate for at least 1 hour, or until completely chilled and gelatin has set.
08 - In a chilled bowl, beat heavy cream, powdered sugar, and vanilla extract until stiff peaks form. Alternatively, use prepared whipped topping.
09 - Spread whipped cream evenly over the chilled cake. Garnish with fresh lime zest and optional lime slices.
10 - Slice cake and serve cold. Refrigerate leftovers.