Preheat oven to 350°F (175°C). Line an 8x8 pan with foil and spray lightly.

In a bowl, whisk melted butter, egg, yolk, brown sugar, vanilla, and sour cream until smooth.

Stir in the brownie mix and salt until just combined; batter will be thick.

Fold in white chocolate chips, reserving a few for the top.

Spread batter into pan, smooth top, sprinkle reserved chips.

Bake 22–28 minutes until edges are set and center is slightly jiggly.

Cool fully in pan before slicing to get neat bars.