In a large pot or saucepan over medium heat add the butter, sugar, buttermilk, vanilla and karo syrup.

Heat until everything is well combined and bring to a boil.

Quickly remove the pot from the heat and add the baking soda, place back over the heat, stirring constantly. The syrup will grow large and threaten to boil over which is why a large pot is essential. If it gets too close to the edge remove from the burner and continue stirring to bring it down.

Stir for 30 seconds over the heat (I often just take mine off so I don't risk boiling over) and serve immediately.