In a 2 1/2 quart saucepan, combine butter, brown sugar, corn syrup and salt.
Bring to a boil over medium-high heat.
Stir constantly to 260 degrees.
Remove from heat and stir in baking soda and vanilla.
Pour over popcorn in large roasting or baking pan.
Stir until all kernels are coated.
Bake at 250 degrees for 1 hour, stirring every 15 minutes.
Spread on wax paper to cool.
Store in a tightly covered container.