Preheat oven to 350 degrees F. Line a 9x9” baking pan with parchment paper and set aside.
In a medium bowl, mix together flour, baking soda, salt, baking powder cinnamon. Set aside.
Then, in the bowl of a standing mixer fitted with the paddle attachment, or in a large bowl with a hand-held mixer, cream together butter and peanut butter until smooth.
Add the granulated sugar and brown sugar and beat until combined.
Add egg, vanilla and milk and beat for 30-60 seconds on medium-high speed until the mixture is light in color and sugar is dissolved.
Add the dry ingredients and mix on low speed until combined.
If desired, add peanut butter chips or desired mix-ins and stir until combined.
Press the dough evenly into the prepared baking dish, then bake in preheated oven for approximately 22-25 minutes until the top is lightly browned.
Let the bars cool in a wire rack (note, they will likely "deflate" a little bit as they cool, that is normal).
Once cooled, cut into 9 or 16 squares and serve.
Video
Notes
Ingredient Substitutions
Salted Butter. Unsalted butter or ghee are good substitutes.
Creamy Peanut Butter. Choose your favorite creamy peanut butter. Just be aware of the salt content. If using unsalted peanut butter, increase the salt by 1/4 teaspoon.
Whole Milk. 2% milk, heavy cream or half and half are good substitutes.
Light Brown Sugar. If you like a little bolder, molasses taste then you can use dark brown sugar.
Granulated Sugar. White sugar or organic cane sugar are both great options.
Sea salt. I only ever bake with pure, fine sea salt. If you are using iodized salt I recommend decreasing the amount of salt by half.
Peanut butter chips. You can omit these, or choose a different mix-in such as chocolate chips, cinnamon chips, etc.
Store/freezeStore these peanut butter cookie bars in an airtight container in the refrigerator for up to 1 week, or in the freezer for up to 2 months.