Melt butter in a medium saucepan over medium heat. Once melted, add the brown sugar and corn syrup. Bring to a boil, stirring constantly. Once boiling, add the baking soda and salt. Stir until combined.
Pour the popcorn kernels into the saucepan and stir until all of the kernels are coated with the caramel mixture. Cover and allow to cook for 2-3 minutes, or until popping slows down.
Once popping has slowed down, remove from heat and pour onto parchment paper or a silicone baking mat. Allow to cool completely before breaking into clusters.