Preheat the oven to 350°F (175°C). Grease an 8×8-inch or 9×9-inch baking pan and line it with parchment paper.
Combine the strawberry cake mix, melted butter, egg, and milk in a large bowl. Mix until a thick dough forms.
Fold in white chocolate chips and diced strawberries (if using).
Transfer the dough to the prepared baking pan and press it evenly into the pan using a spatula or your hands.
Bake for 20-25 minutes, or until the edges are lightly golden and the centre is set. Be careful not to overbake, as these brownies are meant to be soft and chewy.
Let the brownies cool completely in the pan before slicing.