Once the outer edge starts to turn colour you can gently swirl the pan a couple of times to evenly distribute, once again, do not stir. When it becomes a deep amber colour, remove from heat and whisk in heavy cream. The mixture will bubble vigorously. Once it has smoothed out, whisk in butter and salt. Stir in rum. Pour into a sealable jar and allow to cool completely prior to storing in the fridge.