Preheat oven to 350F (175C). Grease a 9x13-inch pan or line with parchment paper. Set aside.
In a large bowl, whisk together butter, sugar, eggs, and vanilla until well-combined.
Whisk in the flour, baking powder, and salt just until combined.
Fold in the chopped chocolate.
Pour the batter into the prepared pan.
Bake until golden brown, about 20-25 minutes. It's better to slighly underbake rather than overbake.
Let the blondies cool in the pan before cutting and serving.
The butter should be just barely melted. I microwave the stick of butter until it's about 80% melted. Then, I whisk it until it's completely melted. You don't want the butter to be hot and sizzling. If you microwave it too long and it is hot and sizzling, let it cool before using. Otherwise, combining hot butter and eggs can give you cooked bits of egg in your batter, and hot butter can also leave you with greasy blondies.
To properly measure the flour, sift or stir the flour to break it up. Lightly spoon into the measuring cup and level off.
Or 1/4 teaspoon table salt.
You can substitute chopped white baking chocolate, white chocolate chips, chocolate chips, or chocolate chunks if you prefer.
You can also add chopped pecans, walnuts, or even chopped macadamia nuts. I use about 1/2 cup of chopped nuts and add them along with the chopped chocolate when using.