Step 1

Prepare and bake cake according to package instructions in a 9x13-inch baking pan.

Step 2

Let cool for 5 minutes, then poke holes all over cake with the handle of a wooden spoon, but don't poke through to the bottom. Set aside.

Step 3

For the filling, add condensed milk, whipping cream and lime zest to a medium bowl. Whisk to combine. Add lime juice and once again, whisk to combine. It will start to thicken quickly; pour over cake, spreading with a spatula to encourage it to fill in holes. Chill for at least 1 hour, and up to overnight.

Step 4

While cake chills, make the topping. To a medium bowl, add whipping cream and vanilla extract. Beat with an electric mixer until starting to thicken, then gradually add powdered sugar and continue beating until stiff peaks form.

Step 5

Spread over cake and garnish with sliced limes and lime zest, if desired. Enjoy!