Pop popcorn and remove old maids. Spread popcorn in a large flat pan and preheat in a 250~ oven. In a saucepan, melt butter and stir in brown sugar, corn syrup, and salt. Stir constantly until the mixture comes to a boil, and continue to boil without stirring for about 5 minutes (240~). Remove from heat and add soda and vanilla, stirring until foamy. If nuts are used, add them now, stirring until heated through and no longer clumped. Pour syrup mixture over preheated popcorn and stir to coat. Bake for about 30 minutes, stirring 2 or 3 times to evenly distribute the coating. Bake longer, about 60 minutes, for a more Crackerjacks-like taste. Cool, break up and store airtight.