Preheat oven to 175°C. Prepare 3 baking trays with baking paper and spray oil.
Combine brown sugar, peanut butter, oil, eggs and vanilla with an electric beater; add water and beat until smooth. Mix all-purpose and whole-wheat flours, baking powder, baking soda and salt in a small bowl. Stir the dry ingredients into the brown-sugar mixture just until combined.
Roll cookie dough into 3cm balls. Place approx 6cm apart on the prepared baking trays. Sprinkle with peanuts, pressing them lightly into the dough with your fingers.
Bake cookies, one sheet at a time for 8 to 10 minutes or until golden brown.. Transfer to a wire rack to cool.