Preheat the oven to 350°F and line an 8x8-inch baking pan with parchment paper, letting the parchment rise up over the side for easy removal after baking. Set aside.
In a large microwave-safe bowl, melt the butter then whisk in the brown sugar and vanilla until smooth. When this mixture is sufficiently cooled, whisk in the egg.
Once the butter mixture is sufficiently cooled, whisk in the egg.
In a separate bowl, whisk together the flour, salt, and baking powder.
Stir this into the wet ingredients until completely combined, then fold in the white chocolate chips and the chopped strawberries. Transfer the batter to the prepared pan.
Bake for 25 to 30 minutes. The middle may still be soft but the edges should be pulling away from the sides and the top should be golden when done.
Allow them to cool completely before removing them from the pan.