Line 9x13 pan with tinfoil, spray tinfoil with cooking spray.
In a large soup pot, melt butter with bag of marshmallows, stirring until melted.
Remove from heat, stir in Nutella and vanilla extract and mix until smooth.
Stir in cocoa krispies until well coated. Press mixture into bottom of prepared pan.
Sprinkle with nuts and marshmallows, pressing lightly into bars. Cool.
Sprinkle chocolate chips over top and press in lightly.
Remove from pan and press into 24 bars. Store in airtight container.
Source: Redbook magazine 2012