Preheat oven to 350°F
Spray a 13X9 pan with nonstick spray. Line the inside of the pan with parchment paper, leaving some to hang over the sides to act as a sling to removed the finished blondies.
Melt butter in a light colored skillet or sauce pan over medium heat, swirling the pan often. (The light color makes it easier to see the browning!) It will become foamy and turn golden as the milk solids begin to toast. Keep swirling! After about 5 - 7 minutes it will start to smell nutty and delicious, and that's your cue to remove it from the heat and pour it into a medium sized bowl to stop the cooking. Allow to cool for a few minutes.
In a small bowl, whisk together your all-purpose flour, baking powder, and salt. Set aside.
Whisk sugars, vanilla and eggs into the cooled brown butter until light and fluffy. Then fold in the flour mixture, until just a few streaks of visible flour remain.
Fold in your add-ins, reserving some to sprinkle on top for the beauty factor.
Spread your mixture into your prepared 13X9 inch pan. You can use your hands or a small offset spatula to coax the batter into the corners, but give them a spritz with nonstick spray first!
Sprinkle the top with your leftover add-ins and bake for 30 - 35 minutes until golden and set, but still slightly gooey in the middle. A toothpick inserted in the middle should come out clean, or with a few moist crumbs.
Allow to cool at least an hour before slicing and devouring.