SPRAY full-sheet pan (or half-sheet pan for trial recipe) with cooking spray. Mix dry cake mix, butter and 1 cup milk (or 1/2 cup milk for trial recipe) in large bowl. Stir in OREO Pieces; press 2/3 onto bottom of prepared pan.
MIX remaining milk and peanut butter until blended; spread onto crust. Top with marshmallows and teaspoonfuls of the remaining cake mix mixture. Sprinkle with nuts and chocolate chips.
BAKE in 300º F-convection oven 20 min. or until center is set. Cool completely before cutting into bars.