Preparation Oil a 9-inch square baking pan. Heat sugar in a dry 4-to 5-quart wide heavy pot over medium heat, stirring with a fork to heat sugar evenly, until it starts to melt. Stop stirring and cook, swirling pot occasionally so sugar melts evenly, until it is a deep golden caramel. Reduce heat to low. Stir in butter, pecans, vinegar, and 1/4 teaspoon salt. When butter is almost melted, add popcorn and stir to coat. Quickly press mixture into baking pan with a spatula. Cool completely. Cut or break into pieces.
Preparation Oil a 9-inch square baking pan. Heat sugar in a dry 4-to 5-quart wide heavy pot over medium heat, stirring with a fork to heat sugar evenly, until it starts to melt. Stop stirring and cook, swirling pot occasionally so sugar melts evenly, until it is a deep golden caramel. Reduce heat to low. Stir in butter, pecans, vinegar, and 1/4 teaspoon salt. When butter is almost melted, add popcorn and stir to coat. Quickly press mixture into baking pan with a spatula. Cool completely. Cut or break into pieces.