In 3-quart saucepan, combine brown sugar, corn syrup, butter, vinegar and salt. Cook and stir until sugar dissolves. Continue to cook until hard ball stage (250 F on candy thermometer).
Pour syrup over popcorn; stir to coat. Add chocolate pieces and nuts; stir just until mixed.
Pour into two 13-by-9-by-2-inch pans, spreading and packing firmly. Cool.
In each pan, cut 12 rectangles. Cut each pint of ice cream into six slices. Sandwich each ice cream slice between two popcorn rectangles.