Line a 9x9-inch pan with parchment paper, leaving an overhang for easy removal. Lightly spray the parchment with cooking spray. Measure out the cereal and set aside.
In a large nonstick pot, melt the butter over low heat. Once melted, add all but 2-1/2 cups marshmallows. Stir constantly over low heat until fully melted.
Remove pot from the heat. Stir in the cocoa powder, chocolate chips, salt, and vanilla extract until smooth. Quickly and gently mix in the Cocoa Krispies cereal. Finally, fold in the reserved 2-1/2 cups of marshmallows until combined.
Pour the mixture into the prepared pan. Lightly spray your hands with cooking spray and gently press the mixture into the pan—avoid pressing too hard to keep the bars soft. Let the bars cool and set for about 15–20 minutes. Once set, use the parchment overhang to lift the bars out and cut them into squares.
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Notes
Note 1: For less sweetness, use half Cocoa Krispies and half regular Rice Krispies. Storage: Rice Krispie Treats are best eaten the day they’re made. If needed, store in an airtight container at room temperature, or freeze for up to 6 months and thaw at room temperature for 15–20 minutes.