1Heat oven to 200°F. Remove any unpopped kernels from popcorn. In very large roasting pan or very large bowl, place popcorn and nuts, or divide popcorn and nuts between 2 ungreased 13x9-inch pans.
2In 2-quart saucepan, melt butter over medium heat. Stir in brown sugar, corn syrup and salt. Heat to boiling, stirring occasionally. Continue cooking 5 minutes without stirring; remove from heat. Stir in baking soda until foamy.
3Pour sugar mixture over popcorn mixture; toss until evenly coated. If using bowl, transfer mixture to 2 ungreased 13x9-inch pans. Bake 1 hour, stirring every 15 minutes. Spread on foil or cooking parchment paper. Cool completely, about 30 minutes.
4In medium bowl, place 3 cups popcorn mixture. In 1-quart saucepan, melt chocolate chips over low heat, stirring constantly. Drizzle chocolate evenly over 3 cups popcorn mixture; toss gently to thoroughly coat popcorn. Spread in single layer on foil or cooking parchment paper. Cool about 2 hours or until chocolate is firm. Add chocolate-covered popcorn mixture to remaining popcorn mixture; toss gently to combine. Store tightly covered.