Oven Made Caramel corn

5 quarts popped corn
1 c butter
2 c brown sugar, firmly packed
1/2 c light corn syrup
1 tsp vanilla
1 tsp salt
1/2 tsp baking soda

spread freshly popped corn on several large shallow sheet pans. keep warm in 250 oven. combine butter,sugar, syrup, salt, and vanilla in a heavy 2 qt. pan. put over med heat, stirring until sugar dissolves. continue boiling to firm ball stage (245*) about 5 min. remove from heat. stir in soda. syrup will foam. pour hot caramel over popcorn in a fine stream. stir to mix well. return to oven for 45 to 50 min, stirring every 15 min. cool slightly. store in airtight containers and set in cool place.