Combine butter, brown sugar, light corn syrup and vanilla in a 2 quart sauce pan. Cook on medium heat until sugar has completely dissolved. Once dissolved and slightly bubbly add the baking soda and salt. This will cause the mixture to foam. Stir well. Pour caramel mixture over the Puffcorn and stir until mixed Place in oven, bake for 45 minutes, stirring every 10 to 15 minutes. Watch closely!
Remove from oven, spread on wax paper and break apart when cool.