Pre-heat oven to 300 degrees and line a large baking sheet with foil. Spray with cooking spray.
In a large bowl, whisk together: oil, honey, cocoa powder, cinnamon, vanilla and salt. Stir in oats, flax seed and walnuts then spread in an even layer on baking sheet. Bake until golden brown and toasted, about 25-30 minutes.
While granola is baking, melt peanut butter in a glass bowl in microwave until thin and creamuy, about 30-45 seconds.
When granola comes out of the oven, place in a large bowl. Drizzle with melted peanut butter and toss well. Let cool, about one hour, before tossing in chocolate chips (if you wish.)