Set the butter in a large saucepot over medium heat. When it's halfway melted, add the sugar and salt. Cook, stirring with a wooden spoon, until the mixture is dark golden brown. It will look grainy and the fat will separate from the sugar and then come back together.
Remove from the heat and add the evaporated milk while whisking. Be careful! The caramel will bubble up. Scrape up any sugar that's stuck to the bottom of the pan. Make sure the sugar is completely dissolved.
Set over low heat and cook, whisking often, until very smooth and creamy, about 1 hour. Remove from the heat and cool completely, stirring occasionally.