Preheat oven to 250 degrees F. Line the bottom and sides of a large roasting pan or two baking sheets with greased foil. Place popcorn in a large heat-proof mixing bowl; set aside.

Meanwhile, melt the brown sugar, butter, corn syrup and salt in a large saucepan over medium-high heat. Bring to a boil and cook for 5 more minutes, stirring constantly. Remove from the heat and carefully stir in the baking soda and vanilla. Pour over the popcorn and stir to coat.

Spread the coated popcorn evenly onto the lined roasting pan or baking sheets. Bake for 45 minutes, stirring every 15 minutes. Remove from oven and let cool. It will become crispy within a few minutes out of the oven. Break into chunks and serve immediately, or store in a ziplock bag for a couple of days.

Source: Adapted from Sing for Your Supper