Preheat oven to 250F.
Line a large roasting pan or deep 9 x13 baking dish with parchment paper (easy clean-up!) and fill with popcorn. Top with almonds. Keep popcorn warm in the oven while preparing the caramel.
Melt the butter in a 3 qt heavy, saucepan over medium heat. Stir in the brown sugar, corn syrup and salt and bring to a boil, stirring constantly.
Once the mixture is at a boil, stop stirring and boil for 4 minutes.
Remove from heat and stir in the vanilla and baking soda.
Pour the mixture in a thin steam over the popcorn. Use tongs to mix the popcorn, almonds and caramel making sure to thoroughly coat.
Put the baking dishes into the oven and let bake for 45 minutes. Every 15 minutes, pull the dishes out and break up the caramel corn using tongs to turn the popcorn over and mixing it up. After 45 minutes, let the caramel corn cool completely before breaking into small pieces.
Store in an airtight container or the popcorn will turn sticky.