Preheat the oven to 350 degrees F. In a large bowl, mix together the cake mix, eggs, vegetable oil and water. Fold in the chopped strawberries.
LIne pan with parchment paper or spray with non stick spray. Pour the batter into a prepared 9 x 13 pan. Bake for approximately 20-25 minutes.
While the cake is baking, blend 1 cup of strawberries in a blender until smooth. Put blended strawberries with sugar and vanilla extract in a pan over medium heat until the sugar has dissolved and the sauce thickens slightly. (could just use a can of sweetened condensed milk instead)
Remove from heat. Once the cake has finished baking, remove and let cool for 10 minutes. Poke the cake all over with the back of a small spoon to make holes (I used a chopstick).
Pour the strawberry sauce all over the cake. Refrigerate for 1 hour. Top with whipped topping with sliced strawberries on top and place in your refrigerator to cool for 1 more hour. Serve!