2. Whisk together flour, baking powder, baking soda, and salt. Set aside.
3. Beat the butter on medium speed until smooth and creamy. Add both sugars and beat for another 3 minutes, or until well incorporated. Add the eggs one by one, beating for one minute after each addition, then beat in the vanilla. Reduce mixing speed to low and add the dry ingredients until they disappear into the batter.
4. Using a rubber spatula, stir in add-ins. Scrape the batter into the buttered pan and use the spatula to even out.
5. Bake for 40 minutes or until a knife inserted into the center comes out clean. They should pull away from the sides of the pan and the top should be a honey-brown. Transfer the pan to a rack and cool for about 15 minutes before turning the blondies out onto another rack. Invert onto a rack and cool the blondies right side up to room temp.