Bake the yellow cake mix following the package instructions in a 9x13 inch baking dish. Allow the cake to cool completely to room temperature before continuing, which ensures the gelatin mixture won't cook the surface.

While the cake cools, stir the lime gelatin into boiling water for 2 minutes until fully dissolved with no granules remaining. Mix in the fresh lime juice. After the cake reaches room temperature, poke holes throughout the surface using a fork, then gradually pour the gelatin mixture over the entire cake so it soaks into the holes. Chill in the refrigerator for at least 2 hours to set the jello and allow the cake to absorb the lime flavor.

Combine the sugar, lime juice, lime zest, and sifted cornstarch in a heavy-bottomed saucepan over medium heat. Whisk steadily for 4 minutes until the mixture reaches a gentle boil and thickens as the cornstarch activates. In a separate bowl, whisk the room-temperature eggs until smooth. Take the pan off the heat and gradually add the eggs to the hot mixture in a steady stream while whisking constantly to prevent scrambling. Place the pan back on medium-low heat and continue cooking for 10 minutes, stirring constantly, until the curd thickens enough to coat the back of a spoon.

Take the lime curd off the heat and stir in the unsalted butter until it melts completely. Add a few drops of gel food coloring to create a vibrant lime green color. Pour the curd into a bowl and let it cool to room temperature, stirring now and then to release heat. Once cooled, refrigerate for at least 2 hours until thoroughly chilled and thickened.

Take the jello-soaked cake out of the refrigerator. Using a rubber spatula, gently fold the chilled lime curd into the whipped topping until fully blended with no streaks. Spread this frosting evenly across the top of the cake, covering it completely. If desired, sprinkle a pinch of salt over the surface to enhance the lime flavor, then garnish with fresh lime zest and thin lime slices.