Preheat oven to 350 degrees F. Grease or line an 8 x 8 baking pan, set aside. In a small mixing bowl combine the flour, baking soda, baking powder, and salt, stir to combine and set aside.
In a large bowl, melt butter. Add in sugars and whisk well to combine, 2-3 minutes. Add in eggs, one at a time, and vanilla. Mix in peanut butter until smooth.
Slowly add in the dry ingredients and mix, the mixture will be fairly thick. Transfer into the prepared baking pan.
Bake at 350 degrees for 25-30 minutes or until a toothpick inserted comes out clean. Remove form the oven and let cool on a wire rack to completely set, sprinkle with flaky salt and allow to cool completely before cutting. These will keep for 4-5 days in an airtight container.
Notes
*If desired substitute crunchy peanut butter, reduce down to 3/4 cup.*Allow blondies to cool completely before slicing.