Coat bottom and sides of foil roasting pan with non-stick spray and put popped corn in the pan, set aside.
In a 2 quart sauce pan slowly melt butter over medium high heat.
Stir in brown sugar, corn syrup and salt.
Bring to a boil stirring constantly.
When a rolling boil starts, boil for 5 minutes without stirring.
Remove from heat and add baking soda and vanilla.
Slowly pour foamy mixture over popped corn, mixing well.
Bake at 250 degrees for 1 hour, stirring every 15 minutes.
Remove from pan onto wax paper to spread out and cool.
Store in tightly sealed container.