01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly.
02 - Combine cake mix, water, vegetable oil, and eggs in a large bowl. Beat according to package directions. Pour batter into prepared pan and bake for 28-32 minutes until a toothpick inserted in the center comes out clean.
03 - Remove cake from oven and let cool in the pan for 10 minutes.
04 - Using the handle of a wooden spoon, poke holes evenly across the entire surface of the cake, spacing them approximately 1 inch apart.
05 - Dissolve lime gelatin completely in 1 cup boiling water. Stir in 1/2 cup cold water and mix well.
06 - Slowly pour the lime gelatin mixture evenly over the warm cake, allowing it to seep into the poked holes.
07 - Refrigerate cake for at least 1 hour to allow gelatin to set completely.
08 - Beat heavy whipping cream with powdered sugar and vanilla extract in a chilled bowl until stiff peaks form, about 3-5 minutes.
09 - Spread whipped cream mixture evenly over the chilled cake. Garnish with fresh lime zest and lime slices. Refrigerate until ready to serve.