Preheat your oven to 350°F (175°C) and grease and flour a 9x13 inch baking pan.
In a large mixing bowl, combine the strawberry cake mix, water, vegetable oil, and eggs. Mix until well combined and smooth.
Pour half of the cake batter into the prepared baking pan, spreading it evenly across the bottom.
In another bowl, beat together the softened cream cheese, powdered sugar, and vanilla extract until smooth and creamy.
Gently fold in the chopped strawberries into the cream cheese mixture.
Spoon dollops of the cream cheese mixture over the cake batter in the pan, and then pour the remaining cake batter on top, spreading it to cover.
Use a knife to swirl the cream cheese filling into the cake batter, creating a marbled effect.
Bake for 35-40 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
Once cooled, drizzle milk over the cake for added moisture if desired, and serve with whipped cream, if using.