FOR THE POPCORN
1/4 cup vegetable oil (not required if air popping with a popcorn maker)
1/3 cup popping corn (10 cups popcorn, Note 1 for lighter caramel coverage)
FOR THE CARAMEL
1 stick butter
1 cup brown sugar , packed
1/2 cup light corn syrup
1/2 tsp salt
1 tsp vanilla extract
1/2 tsp baking soda
Heat oil in a large pot over medium heat.
Cover with a lid. Once the popcorn starts popping in earnest, shake the pot gently once.
Remove from heat when the popping stops (when you can count to 3 between pops) and transfer to a (very!) large bowl. You need 10 cups of popcorn.
Preheat oven to 110°C/230°F.
Melt butter in a saucepaan over medium heat.
Add sugar, corn syrup and salt. Stir until just combined.
When it starts to bubble, simmer for 4 minutes - DO NOT STIR!
Remove from heat then quickly whisk in vanilla and baking soda. When you stop whisking, the caramel will foam and increase in volume.
Quickly pour straight over popcorn, toss until caramel cools and starts to harden.
BAKING TO CRISP:
Spread popcorn on 2 baking trays.
Bake for 30 minutes, tossing every 10 minutes or so. Caramel will remelt for the first 2-3 tosses - tossing coats popcorn more evenly with caramel.
Remove from oven, leave to cool. Gently break into pieces and serve.