Preheat oven to 350℉. Grease an 8x8 baking dish with butter or non-stick cooking spray. Set aside.
In a large mixing bowl, use high speed on an electric mixer or stand mixer fitted with a paddle attachment to cream together the brown sugar and butter until creamy.
Add eggs, vanilla, and peanut butter to the bowl, and mix on medium speed for 2 minutes, until ingredients are completely incorporated.
Add flour, baking powder, baking soda and salt and mix on low speed until just combined. *Don't over mix or your blondies will be tough and crumbly.
Pour the batter into the greased baking dish and bake in preheated oven for 35-45 minutes, until a toothpick inserted into the blondies comes out clean.
Note: The blondies may look underbaked. They are meant to be somewhat soft and gooey, but they will firm up a little bit as they cool.
Notes
Don’t over mix the batter. If the flour is mixed too much, the protein strands (gluten) can cause the bars to be dry and crumbly.
Tap the bottom of the pan on the counter before baking. After you’ve leveled out the batter in the pan, give the pan a few taps on the counter to settle the batter and remove any air bubbles.
Avoid over baking. When a toothpick inserted into the peanut butter blondies comes out clean or with a few moist crumbs, remove the pan from the oven. The bars will finish baking as they cool down after removing them from the oven.