Prepare the yellow cake mix according to package directions, baking it in a standard 9x13 inch baking dish. I always use Duncan Hines because it creates the most moist crumb, which is essential for absorbing the lime liquid in the next step. Let the cake cool completely to room temperature before proceeding—this prevents the jello mixture from cooking the surface.

While the cake cools, dissolve the lime gelatin in boiling water, stirring constantly for 2 minutes until completely dissolved. Stir in the fresh lime juice to create a tangy soaking liquid. Once the cake has cooled completely, use a fork to pierce it all over, creating small holes that will allow the jello mixture to penetrate and keep the cake incredibly moist. Slowly pour the jello mixture over the entire surface, allowing it to soak into the holes. Refrigerate for at least 2 hours so the jello sets and the cake absorbs all the lime flavor.

In a heavy-bottomed saucepan, combine the sugar, lime juice, lime zest, and cornstarch (sifted to remove any lumps) over medium heat. Whisk continuously for 4 minutes until the mixture comes to a gentle boil and the cornstarch fully activates, thickening the base. In a separate bowl, whisk together the room-temperature eggs until well combined. Remove the pan from heat and slowly temper the eggs by whisking them into the hot mixture in a steady stream—this prevents scrambling and creates a silky curd. Return the pan to medium-low heat and cook for 10 minutes, stirring constantly, until the mixture coats the back of a spoon and reaches a custard-like thickness.

Remove the lime curd from heat and stir in the unsalted butter until completely melted and incorporated. Add a few drops of gel food coloring to achieve a vibrant lime green hue—I prefer gel over liquid coloring because it doesn't thin out the curd. Transfer the curd to a bowl and let it cool to room temperature, stirring occasionally to release heat evenly. Once cooled, refrigerate for at least 2 hours until it's chilled and has thickened further.

Remove the jello-soaked cake from the refrigerator. Fold the chilled lime curd from Step 4 into the whipped topping using a rubber spatula, mixing gently until no streaks remain—this creates a light, fluffy frosting that complements the moist cake beautifully. Spread the lime curd frosting evenly over the cooled cake, covering the top completely. Add a pinch of salt to the surface if desired to enhance the lime flavor, then garnish with fresh lime zest and thin lime slices for visual appeal.