Line a large baking sheet with parchment paper. This prevents the sticky toffee from sticking. Set the tray aside for later use. This step saves time later.

Place the butter in a medium saucepan. Turn the heat to medium-low. Let the butter melt slowly and completely. Do not let it burn or brown.

Stir in the brown sugar and corn syrup. Add the salt to the pan now. Stir until the mixture is very smooth. Use a steady medium heat.

Bring the sauce to a gentle boil. Let it bubble for about five minutes. Do not stir it during this time. The sauce will turn dark gold.

Remove the pan from the heat. Stir in the vanilla and baking soda. The sauce will foam up and grow. This makes it easy to coat.

Pour the hot sauce over the popcorn. Stir quickly with your wooden spoon. Coat every piece of corn with toffee. Be careful as the sauce is hot.

Spread the popcorn on the prepared tray. Bake at a low heat for ten minutes. Let it cool completely before you eat. It will become very crunchy.