Place butter, brown sugar, water, and salt in a medium saucepan, 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 occasionally until it is super bubbly.

Remove the saucepan from the heat and drizzle in the evaporated milk and vanilla while continuing to stir.

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