Line a 9" x 9" baking pan with parchment paper and set aside.
In a large saucepan or Dutch oven, melt the butter over low-medium heat.
Add 7 cups of marshmallows, setting aside one cup for later. Immediately stir the marshmallows and melted butter, stirring constantly with a heatproof silicone spatula for a few minutes, until the marshmallows are mostly melted.
Turn off the heat and stir in the vanilla until incorporated.
Then add the Cocoa Krispies cereal and stir constantly to coat.
Gently fold in the remaining 1 cup of marshmallows and the chocolate chips until well combined.
Pour the mixture into the prepared pan and use the silicone spatula to press the mixture into the bottom and corners of the pan until the top is fairly even.
Allow the cocoa krispies to set in the refrigerator for 30 minutes or at room temperature for an hour. Then slice into square or rectangular bars.
Notes
Makes 12 square bars. You can slice them into smaller pieces if you wish. Storage: Store in an airtight container at room temperature for up to 2 days. You can also refrigerate these for up to 5 days, or freeze them for up to one month. If you freeze them, be sure to thaw them out at room temperature overnight.