Preheat oven on 325℉/165℃. Line a 9x13 pan with parchment paper. Dab a tiny bit of butter into the corners and the middle of the pan to hold the parchment paper in place.
In the bowl of a stand mixer using the paddle attachment or in a large mixing bowl using a hand mixer, mix together the sugars and butter on low speed until crumbly. Scrape down the sides of the bowl and the mixer paddle. Add the oil and continue to mix until the mixture is well combined.
½ cup butter, ½ cup white granulated sugar, ½ cup packed brown sugar, 2 Tablespoons avocado oil
Scrape the sides of the bowl again and add the eggs, peanut butter, and vanilla extract. Mix on medium speed until the mixture is completely smooth. This only takes a minute or two.
In a medium size mixing bowl, combine the flour, baking powder, and salt. Mix well. Add half of the flour mixture to the peanut butter mixture in the mixing bowl and mix on low just until incorporated. Scrape the sides of the bowl and the paddle and then add the remaining flour and mix until incorporated.
1¼ cups all purpose flour, 1 teaspoon baking powder, ½ teaspoon fine grind sea salt or kosher salt
Spoon the blondie batter into the pan and spread it evenly in pan. This batter is a little thicker than most brownie batters so it isn't going to pour into the pan and it won't spread as much. Try to get it as evenly spread into the corners as possible.
Bake in a 325℉/165℃ preheated oven for 20-25 minutes. They are done when the top just starts to turn a golden brown. Remove from the oven and let them cool completely on a cooking rack.
Peanut Butter Frosting
In a medium size mixing bowl or using the bowl of your stand mixer with the paddle attachment, mix the butter, peanut butter, vanilla paste, and salt on medium speed until nice and smooth.
½ cup butter, ½ cup creamy peanut butter, 1 teaspoon vanilla paste, ¼ teaspoon fine grind sea salt or kosher salt
Sift the powdered sugar and measure out 1 ⅓ cups. Add the powdered sugar and mix on LOW until it is incorporated and then mix on medium speed about 30-60 seconds or until smooth. Taste the frosting and add more powdered sugar if you like it stiffer and sweeter.
1⅓ cups sifted powdered sugar
Serving the Peanut Butter Blondies
When the blondies are completely cooled, spread the peanut butter frosting over the top and refrigerate for about 30 minutes to set the frosting. Use the parchment paper to remove the blondies from the pan and place them on a cutting board. Cut into the size squares you want and serve!
Nutritional Values are based on a 2" peanut butter blondie. Store any leftover blondies in an airtight container on the refrigerator or you can freeze them for longer storage.