Preheat oven to 350. Line two baking sheets with parchment paper or a silicone baking mat.
Add all-purpose flour, oat flour, ground flaxseeds, baking powder, and salt to a mixing bowl and stir.
In a separate mixing bowl, mix peanut butter and sugar with a handheld beater or whisk until fully combined. Add milk and vanilla, and continue mixing.
Slowly add in dry ingredients. You may have to mix with a fork or your hand, because the mixture will become crumbly.
Form the mixture into a ball, and scoop out balls with a tablespoon. Roll until they’re smooth and place about 2 inches apart from each other on the baking sheets.
Take a fork and lightly press until the balls are flattened, then perpendicularly again to make the classic crisscross pattern.
Bake for 10-15 minutes, until just slightly golden. They should still be slightly soft and will harden as they cool, so don’t remove them from the sheet until after about 10 minutes.
Store in an airtight storage container for up to 5 days, so the cookies stay fresh.