Prepare the white cake according to the package instructions in a 9×13 baking dish (preferable clear) and allow it to cool completely.
When the cake is completely cooled, use a fork or wooden skewer to poke small holes all over the cake.
Poke cakes should have a uniform amount of holes so the key lime filling is able to spread throughout the cake.
In a medium bowl, whisk together the condensed milk, lime juice, whipping cream, lime zest, and green food coloring until just combined.
Spread the key lime mixture over the top of the cake. Cover the pan and place the cake in the refrigerator to chill for one hour.
Remove the key lime poke cake from the fridge and spread the whipped topping over the entire cake. You may not need the whole container to cover the cake. Cover the pan and return the cake to the refrigerator to chill for at least one hour or up to overnight.
Top the whipped cream with lime zest and lime slices before serving.