Melt the butter and unsweetened chocolate together in a large, heavy-bottomed saucepan over low heat, stirring occasionally.
Stir the corn syrup and boiling water together in a heat-proof measuring cup then add to the pan with the sugar.
Increase heat to medium and stir mixture until it reaches boiling.
Boil without stirring for 8 minutes. Really. Do not stir that stuff.
Remove from heat and stir in the vanilla extract and salt. It will bubble up violently, so be careful.
Let rest for 5 minutes before pouring into a heat-proof container with a tight fitting lid. (A canning jar works perfectly for this situation.) Be certain the container you use is small enough to fit into the microwave for reheating purposes.
Leftovers can be stored in the refrigerator.