Wipe out pot you used to pop corn. Add sugar and water to it, and bring to a boil over medium-high heat, only stirring until sugar has dissolved. Boil, swirling pan around (wear potholders) until caramel is a deep amber color, which took about 8 to 9 minutes in my already-heated-from-the-popcorn pot. Remove from heat and stir in butter (mixture will hiss and bubble furiously). Return to stove, reduce heat to medium-low, and cook for another 2 to 3 minutes, removing from the stove if it appears smoky sooner. Remove from heat and whisk (a whisk will best break it up) in the salt, baking soda and miso.