In a medium saucepan over medium heat, combine the butter and brown sugar, stirring until the sugar dissolves.
Add the condensed milk and bring the mixture to a gentle boil, stirring constantly. Add in the salt.
Continue cooking, stirring frequently, until the mixture thickens and turns a light golden caramel colour (about 10–15 minutes).
Remove from the heat and let it cool slightly before transferring it to a heat proof dish. You can use this salted caramel sauce on ice cream, desserts or in coffee for a delicious salted caramel iced latte.