Preheat your oven to 350℉.
Lightly spray an 8x8- or 9x9-inch square pan with cooking spray, then line it with parchment paper. Be sure the parchment paper hangs over the sides of the pan. You'll use the excess as "handles" to lift the granola bars out of the pan later. Set aside.
Add the oats and cashews (if using) to a rimmed baking sheet and place in your preheated oven. Toast for five minutes, then give everything a good stir and return it to the oven. Toast for another five minutes.
Remove the oat mixture from the oven, transfer it to a large bowl and add the peanuts. Set aside.
In a medium sauce pan, melt the butter over medium low heat. Then add the brown sugar and honey. Continue cooking, stirring frequently, until the sugar has melted and the mixture begins to bubble lightly.
Once the sugar has melted and the honey mixture is smooth, turn off the heat and add the peanut butter. Stir until smooth, then add the vanilla and salt. Continue stirring till everything is well incorporated.
Pour the peanut butter-honey mixture over the oats and nuts in the bowl and stir till everything is well coated. (A silicone spatula works great for this step.) You don't want to leave any dry bits, as those won't bind together in the finished bars. Transfer the granola to the prepared baking pan, pressing it down into a firm, even layer. A solid, flat spatula (the kind that doesn't have slots) or a flat-bottomed cup coated with cooking spray work well for pressing. We like to press it extra firm to ensure the bars hold together.
Let the mixture cool for 6-10 minutes, then scatter the chocolate chips over the top. Press them in with same spatula or flat-bottomed cup coated with cooking spray.
Place the pan in the refrigerator for 4-6 hours or overnight.
Once thoroughly chilled, remove the pan from the refrigerator. Lift the entire block of bars out of the pan using the parchment "handles" and place it on a cutting board.
Cut into 16 squares, or any other shape you prefer. Store in an airtight container in a dark, cool pantry for 1-2 weeks, or in the refrigerator for 2-3 weeks.