Step 1: Prepare your pan. Lightly grease an 8x8 inch (20x20 cm) square baking pan with a little oil or cooking spray. Line it with parchment paper, leaving an overhang on two sides. This parchment paper sling will make it much easier to remove the set granola bar slab later.
Step 2: Combine dry ingredients. In a large mixing bowl, combine the rolled oats, chopped nuts (if using), seeds, dried fruit, and a pinch of salt. Give them a good stir to ensure everything is evenly distributed.
Step 3: Prepare the wet mixture. In a small saucepan over medium-low heat, combine the nut butter, honey (or maple syrup), coconut oil, and vanilla extract. Stir constantly until the mixture is smooth, melted, and just starting to simmer around the edges. This should take about 3-5 minutes. Don't boil the mixture, just warm it enough to become fluid and cohesive.
Step 4: Combine wet and dry. Pour the warm wet mixture over the dry ingredients in the large mixing bowl. Using a sturdy spatula or wooden spoon, mix thoroughly until all the dry ingredients are completely coated and no dry patches remain.
Step 5: Press into the pan. Transfer the granola bar mixture to the prepared 8x8 inch pan. Using the back of your spatula, a piece of parchment paper, or even the bottom of a flat glass, press the mixture down firmly and evenly across the entire pan. The harder you press, the better your granola bars will hold together. Aim for a compact, uniform layer.
Step 6: Chill until set. Place the pan in the refrigerator for at least 2 hours, or until the granola bar mixture is completely firm. This chilling time is essential for the coconut oil and nut butter to solidify, ensuring your bars don't crumble when cut.
Step 7: Cut and store. Once firm, use the parchment paper overhangs to lift the entire slab of granola bars out of the pan. Place it on a cutting board. Using a sharp, large knife, cut the slab into 10-12 bars or squares of your desired size. Wipe the knife between cuts if it gets sticky.
Step 8: Store your delicious granola bars. Store the finished granola bars in an airtight container at room temperature for up to 1 week, or in the refrigerator for up to 2 weeks. For longer storage, you can individually wrap them and freeze them for up to 3 months. They thaw quickly at room temperature.