Preheat the oven to 250°F.

Combine the butter, brown sugar, corn syrup and salt in a saucepan. Bring to boil over high heat and cook for 5 min.

Add baking soda. Stir it quickly.

Put popcorn in a roasting pan or baking sheet pan.

Pour the hot butter mixture over the popcorn. Stir well.

Bake and stir every 10 min, until all the popcorn is completely coated and golden brown, about 30 to 40 min.

Let it cool in the pan before storing to a sealed container.