In a large bowl, cream the butter until soft and smooth.
Add the peanut butter and continue to mix until creamy and smooth.
Add the granulated and brown sugar and mix until smooth, light and fluffy.
Add the thickened flax mixture and pure vanilla extract. Blend until smooth, making sure to scrape down the side occassionally.
Add the flour mixture, a small portion at a time, and blend until incorporated and smooth.
Scoop dough and place 1-2 inches apart on the silicone-lined baking sheet. Press down on each cookie with a fork to flatter or make the traditional crisscross pattern.
Place the baking sheet into the oven and bake for approximately 12-14m minutes or until the cookies turn a golden brown.
Once done, take the cookies out of the oven and allow to cool directly on the baking sheet for several minutes.
Once slightly cooled, transfer the cookies to a cooling rack and allow them to cool completely before storing.