Heat oven to 250°F. Spray 2 (13x9-inch) baking pans with no stick cooking spray. Divide popcorn between pans.
Combine brown sugar, sugar, butter and corn syrup in 3-quart saucepan. Cook over medium-high heat, stirring occasionally, until mixture comes to a full boil (7 to 9 minutes). Reduce heat to medium-low. Continue cooking, without stirring, until candy thermometer reaches 234°F or small amount of mixture dropped into ice water forms a soft ball (4 to 6 minutes).
Remove from heat; stir in baking soda and vanilla. Pour caramel mixture over popcorn; sprinkle with nuts. Sprinkle each pan with 1/2 cup chocolate chips. Stir popcorn to coat.
Bake for 45 minutes, stirring every 15 minutes. Immediately place caramel corn on waxed paper. Sprinkle evenly with remaining chocolate chips. Cool completely. Break into pieces. Store in airtight containers.