Combine the sugar and butter in a medium sized saucepan. Melt on medium heat, then bring to a bubbling boil, letting the sugar darken somewhat. Do not boil until it's hard candy however.
Once the butter and sugar have started bubbling, let boil for another 2-3 minutes, then slowly and carefully pour in the hot water. It most likely WILL SPIT so be careful!
Stir together and bring to a boil again.
To thicken, combine the cornstarch and cold water, then whisk into the boiling brown sugar mixture. Cook until it thickens up slightly then remove from the heat.