Place butter, brown sugar, water, and salt in a medium sauce pan, then place the saucepan over medium heat, stirring gently with a wooden spoon until the butter melts.

Let the mixture come to a boil then let simmer for 5-7 minutes, stirring every now and then. It will be super bubbly!

Remove the saucepan from the heat and drizzle in the evaporated milk and vanilla while continuing to stir. (I mix the two ingredients together first, then add them in.)

Let the mixture cool to room temperature. It will thicken as it cools!