Preheat the oven to 350ºF. Line a large baking sheet with parchment paper or a Silpat. Set aside.
In a small bowl, whisk together the ground flax seeds and milk and allow to stand for 5 minutes.
Meanwhile, combine all-purpose flour, whole wheat flour, baking soda, and salt in a large bowl.
In a medium bowl, combine the maple syrup, vegetable oil, peanut butter, and vanilla. Stir vigorously until thoroughly blended. Add the flax mixture and stir until well combined.
Pour the wet ingredients into the dry ingredients, add the chopped peanuts, and mix well.
Using a cookie dough scoop, drop spoonfuls of dough onto the prepared baking sheet, leaving about 2 inches between each cookie. The cookies will puff slightly but won't spread too much. Bake them for 13-15 minutes until lightly browned and set in the middle. Allow to cool on the pan for 5 minutes before transferring them to a wire rack to cool completely.