HEAT oven to 225ºF.
SPRAY 15x10x1-inch pan with cooking spray. Combine popcorn and nuts in large bowl.
BRING chocolate, sugar, corn syrup and butter to boil in saucepan on medium heat, stirring constantly; simmer on medium-low heat 5 min., stirring constantly. Remove from heat; stir in baking soda. Pour over popcorn mixture; toss to evenly coat. Press onto bottom of prepared pan.
BAKE 1 hour. Cool completely. Break into pieces.