Put the cup of water and granulated sugar in a heavy-bottomed saucepan.
Bring to a boil, stirring a couple of times to make sure all the sugar dissolves.
Allow the mixture to boil for just a few seconds, and then remove it from the heat.
Leave the syrup in the pan and keep it convenient to the stove.
Make the Butterscotch
In a heavy-bottomed saucepan that is taller than you think you'll need, combine the dark brown sugar, the ounce of water, butter, and salt.
Stir a couple of times, and then put the lid on the pan.
Heat over medium heat until all the butter has melted and the sugar is dissolved.
Remove the lid and cook the mixture together until it reaches 300-310F on an instant-read thermometer.
Carefully pour about half the simple syrup into the butterscotch, stirring well. Mixture will bubble up a Very Lot, and the butterscotch may harden.
Once the bubbling has calmed down, add the rest of the simple syrup and whisk over low heat until the butterscotch is completely melted.
Whisk in the vanilla off the heat.
Allow the syrup to cool and then store in an airtight container in the fridge for up to 2 weeks.
Warm the syrup before serving and then stir well to incorporate any butter that is floating on top.
Notes
If you're only going to use this syrup in iced coffee, feel free to emulsify the fat into it using a blender or your immersion blender.Store in an airtight container for up to 2 weeks.