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.
03 - Pour batter into prepared pan. Bake for 28 to 32 minutes until a toothpick inserted in the center emerges clean. Remove from oven.
04 - Let cake cool for 15 minutes. Using the handle of a wooden spoon, poke holes evenly across the surface, spaced about 1 inch apart.
05 - Dissolve lime gelatin in 1 cup boiling water, stirring until completely dissolved. Stir in 1/2 cup cold water.
06 - Slowly pour the gelatin mixture over the warm cake, ensuring it seeps into the holes. Allow to cool slightly.
07 - Refrigerate for at least 1 hour until completely chilled and the 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 and decorate with thin lime slices if desired.
10 - Slice into squares and serve cold. Store covered in refrigerator for up to 3 days.