Preheat the oven to 350°F and line a 9x9-inch baking dish with parchment paper (or lightly grease and flour the pan). (You could use a 9x13-inch pan for more servings, but the bars will be thinner.)
In a large microwave-safe bowl, combine the butter and half of the white chocolate chips. Melt in 20 second intervals, stirring in between each time. Continue until the mixture is creamy with no lumps. (This takes about 2 minutes.)
Allow the chocolate mixture to cool slightly (about 5 minutes).
To the melted chocolate mixture, add sugar. Stir well.
Add the eggs and the egg yolk, one at a time, stirring well after each addition.
Stir in the vanilla extract.
In a separate, medium-sized bowl, whisk together the flour, cornstarch, baking powder, and salt.
Gradually stir the flour mixture into the butter and egg mixture until completely combined. Fold in the remaining white chocolate chips.
Spread the batter evenly into the prepared pan. Bake for 35-40 minutes, or until the edges just begin to turn golden brown. (If you are using a 9x13-inch pan, bake for 25-30 minutes.)
Allow to cool before cutting and serving.