In a saucepan over medium-high heat, add your butter, brown sugar, water, and salt.
Bring to a full boil for 5 minutes while stirring occasionally.
in a separate cup, add the vanilla extract to your evaporated milk. That way you can add it all in to the pot at once.
Once your mixture has been boiling for 5 minutes, remove it from the heat and slowly pour in your evaporated milk/vanilla mixture. It's going to bubble up like crazy but just keep stirring until it all comes together.
Make sure you cool it down before serving so it's not piping hot!