Preheat oven to 350 degrees F. Lightly coat a baking sheet (13x18x1) with non stick baking spray and set aside.
Combine the cake mix, vanilla pudding mix, eggs, milk, oil and sour cream in a large bowl and beat on low speed for 30 seconds, then medium speed for 1 1/2 to 2 minutes, scraping the sides of the bowl as needed. Pour the cake mix into the prepared pan.
Bake for 20 to 25 minutes or until toothpick inserted in center comes out clean. Let cool completely.
For the Raspberry Cake Filling:
Combine the frozen raspberries, sugar and 1/2 cup water in a small saucepan. Bring the mixture to a small boil, stirring often to break down the raspberries.
In a separate small dish combine the remaining 1/2 cup water with 4 tbsp cornstarch and pour into the simmering raspberry mixture. Let cook for a few more minutes until the filling starts to thicken. Transfer mixture to a bowl to cool completely. Set in fridge until chilled, then spread mixture evenly over the cooled cake.
For the Cream Cheese Topping:
Add the cream cheese to a large bowl and beat with mixer until light and fluffy. Add the whipped topping and beat in until fully combined.
Dollop the cream cheese mixture over the top of the cake and carefully spread into an even layer.
Top with fresh raspberries as desired and keep chilled until ready to serve.
Cake can be made up to 2 days ahead of time. Cover and store in fridge.