01 -
Preheat oven to 325°F (165°C) and line a 9x9" baking pan with parchment paper and set aside.
02 -
Combine butter and chopped white chocolate pieces in a microwave safe bowl. Heat in 20-second increments, stirring in between, until melted and smooth.
03 -
In a separate bowl, combine sugar, eggs, and egg yolk and whisk vigorously until pale yellow (about 30 seconds, you can also use an electric mixer on high speed for about 15 seconds).
04 -
Stir in vanilla extract then add chocolate mixture and stir until completely combined.
05 -
In a separate bowl, whisk together flour, salt, and baking powder. Stir into white chocolate mixture until thoroughly combined.
06 -
Use a spatula to fold in white chocolate chips and spread the batter evenly into prepared baking pan.
07 -
Transfer to center rack of 325°F (165°C) oven and bake for 42 minutes or until a toothpick inserted in the center comes out clean or with moist crumbs.
08 -
Allow to cool before slicing and serving (otherwise they may appear underbaked/gooey in the center).