Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large mixing bowl, combine the vanilla cake mix, water, vegetable oil, and eggs. Beat with an electric mixer on medium speed for 2 minutes until well blended.
In a separate bowl, mix the chopped strawberries, cinnamon, and brown sugar until combined.
Pour half of the cake batter into the prepared baking pan. Spread it evenly.
Add the strawberry mixture on top of the batter, spreading it evenly. Then pour the remaining cake batter over the strawberry layer. Use a knife to swirl the layers for a marble effect.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Once baked, allow the cake to cool in the pan for 10 minutes, then transfer it to a cooling rack to cool completely.
For the strawberry cream icing, beat the softened butter in a mixing bowl until creamy. Gradually add powdered sugar and mix until smooth.
Add the pureed strawberries and vanilla extract to the icing mixture and beat until well combined. If the icing is too thick, add milk one tablespoon at a time until the desired consistency is reached.
Once the cake is completely cool, spread the strawberry cream icing over the top. Slice into pieces and serve.