Line an 8x8-inch baking dish with parchment paper, leaving extra hanging over the sides for easy removal.
In a large bowl, combine the 2 cups of rolled oats with your chosen ½ cup of mix-ins (chocolate chips, dried fruit, seeds, or nuts). Mix well to evenly distribute.
In a small saucepan over low heat, melt the ½ cup of nut butter and ⅓ cup of honey (or maple syrup/agave) together, stirring until smooth. Add 1 teaspoon of vanilla extract (optional) and a pinch of salt. Stir to combine.
Pour the melted nut butter mixture over the dry oat mixture. Stir until all the oats are fully coated.
Transfer the mixture to the prepared baking dish. Press it down firmly and evenly using the back of a spoon or spatula.
Place the pan in the refrigerator and chill for at least 2 hours, or until the bars are firm enough to cut.
Once chilled, lift the bars out of the pan using the parchment paper. Cut them into 10-12 bars.
Store the granola bars in an airtight container at room temperature for up to 1 week, or refrigerate for longer storage.