Grease 13 x 9 cake pan, and preheat oven to 350 degrees F.

Mix cake according to package directions.

Pour batter into prepared cake pan, and bake as instructed on package.

Allow to cool completely. Then using the handle of a wooden spoon, poke holes half-way through the cake spaced about 1 ½ inches apart.

In a medium mixing bowl, combine sweetened condensed milk, ¾ cup of the heavy cream, and lime zest, and whisk well. Add lime juice, and whisk again. It will start to thicken. Quickly pour over cake, spreading to fill in holes. Cover and chill for several hours or even a few days. It takes time for the lime flavor to infuse the cake.

An hour before serving, sprinkle ½ teaspoon unflavored gelatin ove 1 tablespoon cold water in a small heatproof bowl, and let rest for 5 minutes. Then heat in the microwave for 5 to 10 seconds until the gelatin completely dissolves into a clear liquid. Set aside to cool for no more than 3 minutes while you do the next step.

Quickly start whipping the remaining 1½ cups heavy cream with powdered sugar and vanilla. When soft peaks begin to form, add one or two tablespoons of the cream to the warm gelatin, stirring quickly to temper the gelatin. If you skip this step the gelatin may seize when it hits the cold cream, and you will end up with gummy lumps. Turn the mixer speed to lowest setting, and slowly add the creamy gelatin mixture, whipping it in as you go. Continue beating to stiff peaks

Spread over lime filling, and refrigerate 30 minutes.

Serve, garnished with sliced limes and zest, if desired.