Place the brown sugar, honey, vinegar and salt in a large, heavy-bottomed pot over medium heat and cook, swirling the pot occasionally, until the mixture registers 300 degrees on a candy thermometer.
Immediately remove the pot from the heat and whisk in the baking soda (mixture will foam up and expand to at least 2x volume).
As soon as the baking soda is fully incorporated, gently pour the hot candy mixture into the prepared pan, taking care not to pop the bubbles.
Gently shake/roll the pan to level the mixture as needed, taking care not to pop the bubbles and let out the air.
Allow the mixture to cool until crisp (approx. 2 hours), then break into pieces.
Coat with melted chocolate, if desired, and allow to cool & harden before serving.