Heat oven to 350°F. Line an 8-inch square pan with parchment and lightly grease.
Whisk melted butter, brown sugar, and granulated sugar until smooth and glossy.
Whisk in eggs and vanilla until fully combined and slightly thickened.
In a separate bowl, stir together flour, baking powder, and salt.
Fold dry ingredients into the wet mixture just until no dry spots remain.
Toss raspberries with cornstarch to lightly coat.
Gently fold in white chocolate chips, then the coated raspberries.
Spread batter evenly in the pan. Sprinkle extra chips on top if using.
Bake 28–32 minutes, until edges are set and a tester shows moist crumbs.
Cool completely in the pan. Lift out, slice into squares, and serve.