Make cake as directed on the back of the box for a 9x13" pan.
Let the cake cool for 20 minutes and "poke" the cake with a large fork (I space my pokes about 1/2" apart and do it until the entire surface of the cake is covered).
Mix together the boiling water and Jell-o until the Jell-o is completely dissolved. Pour in the cold water and mix well then pour over the cake.
Cover the cake with plastic wrap and refrigerate for three hours.
Combine the Cool-Whip, cream cheese, powdered sugar, and vanilla and beat until smooth (I have found that it's easiest to use an electric mixer, but you don't have to). Spread on top of cake.
Top with strawberries and keep in the fridge until ready to serve.