Combine 1 cup chocolate morsels, brown sugar, and next 3 ingredients in a heavy saucepan; cook over medium heat, stirring constantly, until mixture come to a boil. Remove from heat, and stir in vanilla and baking soda.
Distribute popcorn and nuts evenly into 2 lightly greased roasting pans. Be sure to remove all unpopped kernels of popcorn before pouring chocolate mixture over popcorn. Pour chocolate mixture evenly over popcorn and nuts, stirring well with a lightly greased spatula.
Bake at 250° for 1 hour, stirring every 15 minutes. Spread on wax paper to cool, breaking apart large clumps as mixture cools. Sprinkle remaining 1 cup chocolate morsels evenly over hot popcorn; let cool. Store in airtight containers.