Make the cake, following the directions on the package.
Let it cool for about 5 minutes.
Poke holes all over it with a chopstick or wooden spoon handle.
Whisk the cream, milk and lime zest for the filling, then mix in the lime juice.
Pour the lime mixture all over the cake, using a spatula to get it in every hole.
Chill overnight if possible or for at least 1 hour.
Beat the cream and vanilla until thick.
Mix in the powdered sugar and whisk until it makes stiff peaks.
Spread the topping over the poke cake and add lime slices to garnish if liked.