Melt the semi-sweet chocolate chips in a microwave-safe bowl in 30-second intervals until smooth (about 2 minutes total). Stir in 1 tablespoon of vegetable oil.
In a very large bowl, combine ¾ of the box of Golden Grahams cereal with the full bag of mini marshmallows.
Pour the chocolate mixture over the cereal and marshmallows. Toss until everything is coated.
Sprinkle the powdered sugar over the top and toss again until evenly coated.
Add the chopped Hershey’s bar, any remaining Golden Grahams cereal, and extra mini marshmallows if using. Toss gently to combine.
Spread the mix on a parchment-lined baking sheet to cool, then package into bags or store in an airtight container.