Get the oven ready by preheating it to 350 degrees F along with a baking pan and baking sheet.
Spread cooking spray on the baking sheet and put it aside.
Now, take a large bowl for making the batter ready.
Cake batter can be prepared according to the package given instructions.
Now, pour the batter into the tray and bake for 35 minutes.
Allow the cake to get completely cool.
Now combine strawberries, water, sugar, lemon juice, 1 teaspoon of lemon zest, and vanilla in a medium saucepan. Adjust the heat to medium and boil the mixture.
Now, let us move on to cooking. Stir gently until the mixture is thick. You can stir for 8 minutes.
Give it cooling time and allow the mixture to get completely cool. See that there are no holes in the thick mixture.
Now, you can pour the strawberry sauce over the mixture using a spoon, preferably a wooden one.
Spread the cool whipped mixture on the top.
Use lemon zest and strawberry slices for garnish.