Set the cider and mulling spices if using in a medium to large sized pot. Boil on medium high for 45-60 minutes until reduced to a thick, sticky syrup (about 1/4 cup).
In a 4 quart pot add the sugar and a squeeze of lemon (no more than a teaspoon). Stir the sugar so that the lemon coats the sugar. Turn the heat on medium. It will melt on the bottom at first, stir it often so it caramelizes evenly.
Heat the heavy cream in the microwave, just warm to touch.
As the sugar melts and darkens, use the back of a wooden spoon to press out any lumps of sugar. Once it has all evenly melted, let it cook for a few minutes until your desired color. The darker the caramel the more bitter the flavor but if you stop it when it's too light it won't have a deep taste; aim for a shade or two darker than amber.
Add the heavy cream – slowly and stirring as you add it so the caramel doesn’t separate. Take care of your hands and use oven mitts, it will spit and sputter and can burn.
Add the butter and stir until it has melted. Turn off the heat then add the cider, salt and vanilla.
Store in an airtight container (it will last a couple of weeks, sealed).