Line a 9-inch square baker with two strips of criss-crossed parchment paper, cut to fit neatly against the base and up the sides. The parchment paper will make it easy for you to slice the bars later.
Place the oats in a large mixing bowl**. Add the cinnamon and salt, and stir to combine. Set aside.
Now we’ll blitz the mix-ins briefly in the food processor or blender (or, you can chop them by hand). Add any large nuts (like almonds or pecans) first and blitz for a few seconds. Then add the rest and run the machine for a few more seconds, until the ingredients are all broken up into pieces smaller than your pinky nail. Pour the mix-ins into the bowl of oats.
In a 2-cup liquid measuring cup, measure out the nut butter. Top with ½ cup honey and the vanilla extract. Stir until well blended. If you must, you can gently warm the liquid mixture on the stovetop or in the microwave, but make sure it’s close to room temperature before you pour it into the dry mixture (this is especially important if using chocolate, since it will melt).
