Preheat oven to 350? F (177? C). Butter an 8-inch-square baking pan. In a large bowl, stir together brown sugar, oil and egg until smooth. Mix in nuts and vanilla. Mix together ground popcorn, baking powder and salt. Add to the oil mixture, stirring well. Spread evenly in the buttered pan. Bake for 20 minutes or until browned. Cut into squares while warm. Makes 16 brownies.